KS3 Science Study Pack: Atoms, Elements and Compounds

Key Knowledge

Chemistry is the study of substances: what they are made from, how they behave, and how they can change into new substances. To understand chemistry, you need to understand atoms, elements, compounds and mixtures.

Matter is anything that has mass and takes up space. Air, water, metal, plastic, salt, sugar, soil and living things are all matter. All matter is made from tiny particles called atoms. Atoms are much too small to see with the naked eye. They are also too small to see using ordinary school microscopes. Scientists use models and powerful specialist instruments to help them understand atoms.

An element is a pure substance made from only one type of atom. Copper is an element because it contains only copper atoms. Oxygen is an element because it contains only oxygen atoms. Carbon, iron, sulfur, helium and aluminium are also elements.

A compound is a pure substance made when atoms of two or more different elements are chemically joined together. Water is a compound because it contains hydrogen atoms and oxygen atoms chemically joined in a fixed ratio. Carbon dioxide is a compound because it contains carbon and oxygen atoms chemically joined together.

A mixture contains two or more substances that are together but not chemically joined. Air is a mixture of gases. Sea water is a mixture of water, dissolved salts and other substances. Soil, fizzy drinks, breakfast cereal and brass are also mixtures.

What Are Atoms?

Atoms are the basic building blocks of matter. Every solid, liquid and gas is made from atoms. Atoms are not tiny versions of the material they make. One copper atom is not a tiny piece of copper wire with the same colour, shininess and bendiness. The properties we can see, such as colour, hardness and melting point, come from huge numbers of atoms arranged and joined in particular ways.

Atoms are shown in particle diagrams as circles or simple symbols, but these diagrams are models. They are not drawn to scale. In real life, atoms are incredibly small and there is space inside and between them. A particle diagram is useful because it helps us think about:

• whether particles are all the same or different
• whether atoms are joined together
• whether a substance is an element, compound or mixture
• what changes during a chemical reaction

Atoms are conserved in chemical reactions. This means atoms are not created or destroyed in ordinary chemical reactions. Instead, atoms are rearranged to make new substances. For example, when magnesium reacts with oxygen, magnesium atoms and oxygen atoms join to form magnesium oxide. The atoms are still present, but they are arranged in a new compound.

A Simple Model of Atomic Structure

At KS3, you should know a simple model of the atom. An atom has a tiny central nucleus. The nucleus contains protons and neutrons. Electrons are found around the outside of the nucleus.

• Protons have a positive charge.
• Neutrons have no charge.
• Electrons have a negative charge.

This simple model helps explain that atoms have smaller parts inside them, but you do not need to learn detailed electron shell arrangements for this topic. That is studied later.

        electron (-)
             o

      [ nucleus ]
      protons (+)
      neutrons (0)

             o
        electron (-)

The diagram is a model, not a real picture. The nucleus is not really a square box, and the electrons do not look like small circles. The model is useful because it labels the main parts and their charges.

Observation and Model

In science, a model is a simplified way to represent something difficult to observe directly. Atoms cannot be seen with ordinary school microscopes, so scientists use models to explain and predict behaviour. A good model is useful, but it has limits. The simple atom model above does not show the true size of the nucleus compared with the whole atom, and it does not show how electrons really behave.

Elements and Chemical Symbols

An element is a pure substance made from only one type of atom. There are over 100 known elements, and they are arranged in the periodic table. Each element has a name and a chemical symbol.

A chemical symbol is a short way to represent an element. Some symbols are one capital letter. Others are one capital letter followed by one lower-case letter.

Element	Symbol	Example use
Hydrogen	H	Fuels and water
Oxygen	O	Respiration and burning
Carbon	C	Living things and fuels
Iron	Fe	Structures and steel
Sodium	Na	Compounds such as sodium chloride
Chlorine	Cl	Water treatment and salt compounds
Copper	Cu	Electrical wires
Helium	He	Balloons and airships

Capital letters matter in chemical symbols. Co means cobalt, which is an element. CO means carbon and oxygen together in a formula. Cl means chlorine, but CL is not the correct symbol for chlorine. The first letter is always a capital letter. If there is a second letter, it is lower case.

Elements can exist in different forms. Some elements are made from single atoms. Helium gas is made from separate helium atoms. Some elements are often found as molecules. Oxygen gas is usually O2, which means two oxygen atoms joined together. Hydrogen gas is H2. These are still elements because only one type of atom is present.

Molecules: Atoms Joined Together

A molecule is a group of atoms chemically joined together. Molecules can be made from atoms of the same element or atoms of different elements.

Oxygen gas, O2, is a molecule of an element. It contains two oxygen atoms joined together. Hydrogen gas, H2, is also a molecule of an element.

Water, H2O, is a molecule of a compound. It contains hydrogen atoms and oxygen atoms chemically joined together. Carbon dioxide, CO2, is another molecule of a compound. It contains carbon and oxygen atoms.

This is an important point: a molecule is not always a compound. A compound must contain atoms of at least two different elements. O2 is a molecule but not a compound because it contains only oxygen atoms.

Compounds: Different Elements Chemically Joined

A compound is a pure substance made when atoms of two or more different elements are chemically joined. The atoms in a compound are joined in a fixed ratio. Water always contains hydrogen and oxygen in the ratio 2 hydrogen atoms to 1 oxygen atom in each water molecule. Carbon dioxide always contains 1 carbon atom and 2 oxygen atoms in each carbon dioxide molecule.

Compounds usually have properties that are very different from the elements they are made from. Sodium chloride is table salt. It contains sodium and chlorine. Sodium is a reactive metal, and chlorine is a poisonous gas, but sodium chloride is a white crystalline solid used in food. The compound is not a simple mixture of sodium and chlorine; it is a new substance with its own properties.

Another example is iron sulfide. Iron is a grey magnetic metal. Sulfur is a yellow non-metal powder. If iron and sulfur are mixed together, they can still be separated with a magnet. If they are heated strongly, they react to form iron sulfide, a compound. Iron sulfide has different properties and cannot be separated with a magnet in the same simple way.

Word Equations

A word equation shows the reactants and products in a chemical reaction. Reactants are the starting substances. Products are the substances made.

Example:

magnesium + oxygen -> magnesium oxide

Magnesium and oxygen are reactants. Magnesium oxide is the product. A new compound forms because magnesium atoms and oxygen atoms become chemically joined. The atoms have not disappeared. They have been rearranged.

Another example:

iron + sulfur -> iron sulfide

Before heating, iron and sulfur are just mixed. After reacting, iron sulfide is formed.

Mixtures: Substances Together but Not Chemically Joined

A mixture contains two or more substances that are together but not chemically joined. The substances in a mixture keep many of their own properties. They are not always present in a fixed ratio, and they can often be separated using physical methods.

Air is a mixture because it contains nitrogen, oxygen, argon, carbon dioxide and other gases. These gases are not chemically joined together in one fixed formula. The amount of water vapour in air can change, which is another clue that air is a mixture.

Salt water is a mixture because sodium chloride is dissolved in water. The salt and water are not chemically joined into a new compound. The water can be evaporated, leaving salt behind. Sand and salt can be separated by dissolving the salt in water, filtering out the sand, then evaporating the water.

Common physical separation methods include:

• filtering, used when an insoluble solid is mixed with a liquid
• evaporation, used to separate a dissolved solid from a solution
• using a magnet, used to separate magnetic materials such as iron from non-magnetic materials
• sieving, used to separate solids with different particle sizes
• chromatography, used to separate some dissolved coloured substances

Physical separation methods work for mixtures because the substances are not chemically joined. They do not separate compounds into their elements in the same simple way.

Comparing Elements, Compounds and Mixtures

Type of substance	What it contains	Are atoms chemically joined?	Fixed composition?	How it can be separated	Examples
Element	One type of atom only	Sometimes atoms are joined to atoms of the same element	Yes, one element only	Cannot be split into simpler substances by ordinary chemical reactions	Copper, oxygen, iron, helium
Compound	Atoms of two or more different elements	Yes, different elements are chemically joined	Yes, atoms are joined in a fixed ratio	Needs chemical reactions to break it into simpler substances	Water, carbon dioxide, sodium chloride, magnesium oxide
Mixture	Two or more substances together	Not all chemically joined into one fixed substance	No, proportions can vary	Often separated by physical methods	Air, sea water, soil, sand and salt

The word pure has a special meaning in chemistry. A pure substance contains only one substance. An element can be pure, and a compound can also be pure. Pure does not always mean safe, clean or natural. Pure chlorine gas is dangerous. Pure sodium metal is reactive. In chemistry, pure means one substance only.

Reading Simple Chemical Formulae

A chemical formula shows which atoms are present and how many of each type are in one particle or unit of a substance.

The small number in a formula applies only to the element immediately before it. If there is no small number, it means there is one atom of that element.

Worked Example: H2O

Formula: H2O

Step 1: Identify the symbols.

• H means hydrogen.
• O means oxygen.

Step 2: Look at the small number.

• The 2 belongs to H, so there are two hydrogen atoms.
• There is no small number after O, so there is one oxygen atom.

Step 3: State the meaning.

One water molecule contains two hydrogen atoms and one oxygen atom. Water is a compound because it contains atoms of different elements chemically joined.

Worked Example: Counting Atoms

Formula	How to read it	Atoms shown	Element or compound?
CO2	C is carbon; O2 means two oxygen atoms	1 carbon, 2 oxygen	Compound
CH4	C is carbon; H4 means four hydrogen atoms	1 carbon, 4 hydrogen	Compound
NH3	N is nitrogen; H3 means three hydrogen atoms	1 nitrogen, 3 hydrogen	Compound
NaCl	Na is sodium; Cl is chlorine	1 sodium, 1 chlorine in the simplest ratio	Compound
O2	O2 means two oxygen atoms joined	2 oxygen	Element
H2	H2 means two hydrogen atoms joined	2 hydrogen	Element

Formula Interpretation Diagram

CO2

C  = carbon atom
O2 = two oxygen atoms

O=C=O  shows one carbon joined to two oxygen atoms

This diagram is a simple model. It helps show which atoms are joined, but it does not show the full size, colour or detailed shape of a carbon dioxide molecule.

Formula Interpretation Table

Formula	Elements present	Number of atoms shown	Element or compound?
H2	Hydrogen	2 hydrogen atoms	Element
O2	Oxygen	2 oxygen atoms	Element
H2O	Hydrogen and oxygen	2 hydrogen, 1 oxygen	Compound
CO2	Carbon and oxygen	1 carbon, 2 oxygen	Compound
CH4	Carbon and hydrogen	1 carbon, 4 hydrogen	Compound
NH3	Nitrogen and hydrogen	1 nitrogen, 3 hydrogen	Compound
MgO	Magnesium and oxygen	1 magnesium, 1 oxygen	Compound
NaCl	Sodium and chlorine	1 sodium, 1 chlorine in the simplest ratio	Compound

Remember: a formula tells the types and numbers of atoms. It does not tell you the colour, state, safety, exact size or full shape of a substance.

Particle Diagrams for Atoms, Elements, Compounds and Mixtures

Particle diagrams use simple shapes or symbols to represent particles. They help classify substances.

Element: one type of atom
O   O   O   O

Element: molecules of one type of atom
O-O   O-O   O-O

Compound: different atoms chemically joined
H-O-H   H-O-H   H-O-H

Mixture: different particles not all chemically joined
O-O   H-O-H   Ar   O-O

How to Classify a Particle Diagram

Ask these questions:

1. Is there only one type of atom?
2. Are atoms joined together?
3. Are all particles identical?
4. Are different substances present together?

Use this logic:

• Single identical atoms = element.
• Identical molecules made from one type of atom = element.
• Identical particles containing different atom types joined together = compound.
• Different particles together but not all joined in one fixed pattern = mixture.

Particle Diagram Classification Task

The following diagrams use letters to represent different atom types. Joined letters show atoms chemically joined.

Box A:   X   X   X   X

Box B:   X-X   X-X   X-X

Box C:   X-Y   X-Y   X-Y

Box D:   X   Y   X   Y

Box E:   X-Y   X   Y-Y   X-Y

Box F:   X-Y-Z   X-Y-Z   X-Y-Z

Box G:   Y-Y   Z-Z   Y-Y   Z-Z

Box H:   X-X   X-X   Y   Y

Classify each box as an element, compound or mixture. Justify each answer using the words atom, molecule, element, compound and mixture where suitable.

Hints:

• Box A contains single identical atoms.
• Box B contains molecules, but all atoms are the same type.
• Box C contains identical molecules made from two different atom types.
• Box E contains more than one type of particle.

The Periodic Table: Symbols, Rows, Columns and Patterns

The periodic table arranges all known elements in a pattern. It is not a random list. Each element has its own place.

An element tile usually shows:

• the element name
• the chemical symbol
• the atomic number

At KS3, the atomic number can be used as a unique number for each element. You do not need to use it for calculations in this topic.

Worked Example: Reading a Periodic Table Tile

   6
   C
 carbon

• 6 is the atomic number.
• C is the chemical symbol.
• Carbon is the element name.

The symbol C has one capital letter. It represents carbon atoms. Carbon is found in living things, fuels, carbon dioxide and many materials.

Rows and Columns

The columns in the periodic table are called groups. Elements in the same group often have similar properties. This is a pattern you should recognise at KS3, without needing to explain it using electron arrangements.

The rows in the periodic table are called periods. Moving across a period, the elements change in a pattern.

Metals are mostly found on the left and in the centre of the periodic table. Non-metals are mostly found on the right. Hydrogen is a non-metal but is often shown on the left because of the table layout.

Simplified periodic table pattern

[ metals mostly here             ][ non-metals mostly here ]
[ groups are columns             ][                    He  ]
[ periods are rows               ][ C  N  O  F  Ne        ]

This is a schematic diagram, not the full periodic table.

Periodic Table Basics Table

Feature	Meaning	Example
Symbol	Short chemical code for an element	O means oxygen
Name	Full name of the element	Oxygen
Atomic number	Unique number for each element	Carbon has atomic number 6
Group	A column in the periodic table	Helium and neon are in the same group
Period	A row in the periodic table	Carbon, nitrogen and oxygen are in the same period
Metal region	Broad area mostly on the left and centre	Iron, copper, aluminium
Non-metal region	Broad area mostly on the right	Oxygen, sulfur, chlorine

Periodic Table Extract

Use this simplified extract to answer the questions later in the pack.

Atomic number	Symbol	Name	Group shown	Period shown	Type	Simple property or use
1	H	Hydrogen	1	1	Non-metal	Gas; used in some fuels
2	He	Helium	18	1	Non-metal	Unreactive gas; balloons
3	Li	Lithium	1	2	Metal	Soft metal; batteries
4	Be	Beryllium	2	2	Metal	Lightweight metal
5	B	Boron	13	2	Metalloid	Used in some glass
6	C	Carbon	14	2	Non-metal	Found in living things
7	N	Nitrogen	15	2	Non-metal	Main gas in air
8	O	Oxygen	16	2	Non-metal	Respiration and burning
9	F	Fluorine	17	2	Non-metal	Compounds in toothpaste
10	Ne	Neon	18	2	Non-metal	Unreactive gas; lights
11	Na	Sodium	1	3	Metal	Found in salt compounds
12	Mg	Magnesium	2	3	Metal	Burns with bright flame
13	Al	Aluminium	13	3	Metal	Cans, foil and aircraft
14	Si	Silicon	14	3	Metalloid	Computer chips and glass
15	P	Phosphorus	15	3	Non-metal	Fertilisers and matches
16	S	Sulfur	16	3	Non-metal	Yellow solid
17	Cl	Chlorine	17	3	Non-metal	Water treatment
18	Ar	Argon	18	3	Non-metal	Unreactive gas in air

Questions to think about:

• Which elements are in group 1 in this extract?
• Which elements are in period 2?
• Which elements are metals?
• Which elements are non-metals?
• What pattern can you see in group 18?

Metals, Non-Metals and Everyday Uses

Metals and non-metals often have different properties. Many metals are shiny, good conductors of electricity and heat, and can be bent or shaped. However, not all metals are magnetic. Iron is magnetic, but copper and aluminium are not.

Many non-metals are poor conductors of electricity. Some are gases at room temperature, such as oxygen, nitrogen and chlorine. Some are solids, such as carbon and sulfur.

Element	Symbol	Metal or non-metal?	Useful properties	Everyday uses
Oxygen	O	Non-metal	Supports respiration and burning	Breathing, hospitals, combustion
Carbon	C	Non-metal	Forms many substances; found in living things	Fuels, pencils as graphite, living organisms
Iron	Fe	Metal	Strong, magnetic, can form steel	Buildings, bridges, tools
Copper	Cu	Metal	Good electrical conductor, not magnetic	Electrical wires, pipes
Aluminium	Al	Metal	Low density, corrosion-resistant	Drinks cans, foil, aircraft parts
Sulfur	S	Non-metal	Yellow solid, forms compounds	Fertilisers, some medicines, matches
Helium	He	Non-metal	Very unreactive, low density gas	Balloons, airships, cooling scientific equipment
Chlorine	Cl	Non-metal	Reactive gas; kills microorganisms in water	Water treatment, compounds in PVC

Chemical Change Compared with Physical Separation

A physical change or physical separation does not make a new substance. Dissolving salt in water is a physical change because the salt and water can be separated by evaporation. Filtering sand from water is physical separation because no new substance forms.

A chemical change makes one or more new substances. Heating iron and sulfur strongly can form iron sulfide. Evidence for a chemical change might include:

• colour change
• heat or light given out
• gas produced
• a new solid formed
• properties changing so the product behaves differently

Observations are what you notice directly. Conclusions are explanations based on evidence.

Observation: The yellow sulfur and grey iron changed into a black solid after heating.

Conclusion: A new substance formed, so a chemical reaction occurred.

Iron and Sulfur Case Study

Before heating: mixture
Fe   S   Fe   S   Fe   S

After heating: compound
Fe-S   Fe-S   Fe-S

Before heating, iron and sulfur are not chemically joined. The iron can be attracted by a magnet. After heating, iron sulfide forms. It has different properties, and the iron cannot be separated with a magnet in the same simple way.

Substance	Colour and appearance	Magnetic behaviour	After heating?	Classification
Iron	Grey metallic filings	Attracted to a magnet	Not heated alone in this test	Element
Sulfur	Yellow powder	Not attracted to a magnet	Melts and reacts when heated with iron	Element
Iron and sulfur mixture	Grey and yellow speckled mixture	Iron part attracted to a magnet	Before strong heating	Mixture
Iron sulfide	Black solid	Not separated by a magnet in the same simple way	Formed after heating	Compound

Safety note: Heating iron and sulfur should be teacher-led or carefully controlled. Wear eye protection, avoid inhaling fumes, and handle hot equipment safely.

Evidence Table: Physical Mixing and Chemical Reaction

Example	Observation	Does a new substance form?	Is separation easy?	Best description
Iron filings mixed with sulfur powder	Grey and yellow particles are together	No	Yes, iron can be removed with a magnet	Physical mixing
Iron and sulfur heated strongly	A black solid forms	Yes	No, not by simple magnet separation	Chemical reaction
Salt dissolved in water	Salt seems to disappear into clear liquid	No	Yes, evaporation can recover salt	Physical change
Magnesium burning in oxygen	Bright light and white powder formed	Yes	No, magnesium oxide is a new compound	Chemical reaction
Sand mixed with water	Cloudy mixture with solid particles	No	Yes, filtration can remove sand	Physical mixing

Separation Evidence Task

Choose a physical separation method for each mixture and explain why it works.

Mixture	Suggested method	Why it works
Salt water	Evaporation	Water evaporates, leaving dissolved salt behind
Sand and salt	Dissolve, filter, then evaporate	Salt dissolves in water, sand does not; filter removes sand; evaporation recovers salt
Iron filings and sulfur	Magnet	Iron is magnetic, sulfur is not
Sand and water	Filtration	Sand particles are insoluble and too large to pass through filter paper
Breakfast cereal with raisins	Hand sorting or sieving	Different pieces are large enough to separate physically

Limitations can affect separation. Very small particles may pass through filter paper. Some salt may be lost during transfer. A magnet may not remove every iron filing if the mixture is thick or the particles are trapped.

Data Task: Gases in Dry Air

Air is a mixture of gases. The approximate percentages of gases in dry air are shown below.

Gas	Percentage in dry air
Nitrogen	78
Oxygen	21
Argon	0.9
Carbon dioxide	0.04

Simple bar chart:

Percentage of gases in dry air

Nitrogen       | ############################################################## 78%
Oxygen         | ##################### 21%
Argon          | # 0.9%
Carbon dioxide | . 0.04%

Questions:

1. Which gas is most abundant in dry air?
2. Which gas shown is least abundant?
3. Why is air a mixture rather than a compound?
4. Oxygen is needed for respiration and burning. Does the chart show that oxygen is the main gas in air?
5. Give one limitation of this data.

Model answers:

1. Nitrogen is most abundant because it makes up about 78% of dry air.
2. Carbon dioxide is least abundant in the table because it is about 0.04%.
3. Air is a mixture because it contains several gases that are not chemically joined in one fixed formula.
4. No. Oxygen is important, but nitrogen has a higher percentage than oxygen.
5. The data is for dry air, so it does not include water vapour. The percentage of water vapour can vary.

Working Scientifically: Classifying Substances from Evidence

Scientists classify substances using evidence. They do not rely only on appearance. Two substances may look similar but have different particle arrangements. Salt water and pure water can both look clear, but salt water is a mixture and water is a compound.

Practical Planning Task

Investigation question: Can different samples be classified as pure substances or mixtures using observations and simple separation tests?

Possible samples:

• copper wire
• salt water
• sand and salt
• iron filings and sulfur powder
• pure water
• carbon dioxide gas in a sealed container or teacher demonstration

Possible tests:

• observe appearance with a hand lens
• test with a magnet
• filter after adding water
• evaporate a small amount of solution
• compare boiling point information from a data source

Variables:

• Independent variable: the sample being tested.
• Dependent variable: the observation or separation result, such as whether a solid is removed by filtration or whether residue remains after evaporation.
• Control variables: sample mass, volume of water added, filter paper type, heating time, apparatus, and observation method.

Fair test ideas:

• Use the same mass of each solid sample.
• Use the same volume of water when dissolving samples.
• Heat samples for the same time when comparing evaporation results.
• Use the same type of filter paper.
• Record observations in the same table.

Reliability means the results are trustworthy and consistent. A test can be made more reliable by repeating it or comparing results with other groups. Repeatability means the same person or group can repeat the method and get similar results. Accuracy means how close a measurement is to the true value. Precision means how close repeated measurements are to each other or how small the measuring scale is.

Evaluation:

• Some mixtures may look like pure substances if the particles are very small or dissolved.
• Some separation may be incomplete.
• Judging colour changes can be subjective.
• Heating can be unsafe unless carefully controlled.
• A single test may not provide enough evidence to classify every sample.

Conclusion-Writing Task: Iron and Sulfur

A class observes iron filings and sulfur powder before and after heating.

Stage	Observation
Before heating	Grey iron filings and yellow sulfur powder can be seen
Magnet test before heating	Some grey particles move towards the magnet
During heating	The mixture glows and becomes very hot
After heating	A black solid is formed
Magnet test after heating	The black solid is not separated by the magnet in the same simple way

Write a conclusion explaining whether a chemical change occurred.

Model conclusion:

A chemical change occurred because a new substance formed. Before heating, the iron and sulfur were a mixture because the grey iron particles and yellow sulfur particles were together but not chemically joined. The magnet attracted the iron, showing that the iron still had its own properties. During heating, the substances reacted. After heating, a black solid formed and it could not be separated by a magnet in the same simple way. This evidence shows that iron sulfide, a compound, formed.

Common Misconceptions

Misconception	Correct idea
Atoms can be seen with the naked eye.	Atoms are far too small to see with the naked eye or ordinary school microscopes.
Atoms are tiny versions of the material they make.	Atoms are particles. The visible properties of materials come from many atoms arranged and joined together.
Elements are always single atoms.	Some elements, such as oxygen and hydrogen, are often found as molecules, O2 and H2.
Every molecule is a compound.	O2 is a molecule but not a compound because it contains only oxygen atoms.
A compound is the same as a mixture.	In a compound, atoms of different elements are chemically joined in a fixed ratio. In a mixture, substances are not chemically joined.
A mixture is a new substance if it looks even throughout.	A mixture can look even but still contain substances that are not chemically joined, such as salt water.
Co and CO mean the same thing.	Co is cobalt. CO contains carbon and oxygen. Capital letters matter.
The small number in a formula applies to the whole formula.	A small number applies only to the element immediately before it at this level.
A formula shows the exact shape and colour of a substance.	A formula shows the types and numbers of atoms, not colour, size or full shape.
Compounds have the same properties as their elements.	Compounds usually have different properties from the elements they contain.
The periodic table is random.	The periodic table is arranged in rows and columns with patterns.
All metals are magnetic.	Iron is magnetic, but copper and aluminium are not.
Air is a compound.	Air is a mixture because its gases are not chemically joined in one fixed ratio.
Pure means safe or natural.	In chemistry, pure means one substance only.
Filtering and evaporation can split compounds into elements.	These methods separate mixtures, not compounds in the same simple way.

Key Vocabulary for Atoms, Elements and Compounds

Term	Student-friendly definition	Example
Atom	A tiny particle that makes up matter	A carbon atom
Element	A pure substance made from only one type of atom	Copper, oxygen, helium
Compound	A pure substance made from atoms of different elements chemically joined	Water, carbon dioxide
Molecule	A group of atoms chemically joined together	O2, H2O, CO2
Mixture	Two or more substances together but not chemically joined	Air, sea water
Chemical symbol	A one- or two-letter code for an element	O for oxygen, Fe for iron
Formula	A chemical code showing the types and numbers of atoms	H2O, CO2
Periodic table	A chart that arranges all known elements in patterns	Carbon is found as C
Metal	An element often shiny, bendy and a good conductor	Iron, copper
Non-metal	An element that usually does not have typical metal properties	Oxygen, sulfur
Nucleus	The central part of an atom	Contains protons and neutrons
Proton	Positively charged particle in the nucleus	Shown as +
Neutron	Particle with no charge in the nucleus	Shown as 0
Electron	Negatively charged particle around the nucleus	Shown as -
Reactant	A starting substance in a chemical reaction	Magnesium in magnesium + oxygen
Product	A substance made in a chemical reaction	Magnesium oxide
Physical separation	Separating substances without making a new substance	Filtration, evaporation
Chemical change	A change where a new substance forms	Burning magnesium
Fixed ratio	A set proportion of atoms in a compound	H2O has 2 H atoms to 1 O atom
Observation	Something noticed or measured directly	The solid turned black
Conclusion	An explanation based on evidence	A compound formed

Real-World Examples

Water is a compound needed for life. Each water molecule contains hydrogen and oxygen atoms chemically joined. Water has different properties from hydrogen gas and oxygen gas.

Oxygen is an element involved in respiration and burning. Humans and many other organisms use oxygen in respiration to release energy from food. Oxygen also helps fuels burn, but oxygen itself is not the fuel.

Carbon is an element found in all living things. It is also found in fuels, carbon dioxide, plastics and many other substances. Carbon can exist in different forms, including graphite and diamond.

Carbon dioxide is a compound linked to breathing out, burning fuels and climate change. It contains carbon and oxygen atoms. It is not a mixture of carbon powder and oxygen gas.

Copper wire is an element used in electrical circuits because copper is a good conductor of electricity. Copper is not magnetic, which helps correct the misconception that all metals are magnetic.

Helium is an element used in balloons because it is less dense than air and very unreactive. Helium atoms are not joined into molecules under normal conditions.

Chlorine is an element used in water treatment because it kills microorganisms. Chlorine gas is dangerous, but chlorine atoms can also be part of safer compounds, such as sodium chloride.

Air is a mixture of gases. It is important for life, weather and combustion, but it is not a pure substance. Sea water is also a mixture. It contains water, dissolved salts and small amounts of other substances.

Worked Examples

Worked Example 1: Is Oxygen Gas an Element or Compound?

Oxygen gas has the formula O2.

Step 1: Identify the atoms.

O means oxygen. The formula contains only oxygen atoms.

Step 2: Look at whether different elements are present.

There is only one element present.

Step 3: Decide.

Oxygen gas is an element. It is also made of molecules because oxygen atoms are joined in pairs. It is not a compound because there are not two different elements chemically joined.

Worked Example 2: Is Water an Element, Compound or Mixture?

Water has the formula H2O.

Step 1: Identify the elements.

H is hydrogen and O is oxygen.

Step 2: Count the atoms.

There are two hydrogen atoms and one oxygen atom in each water molecule.

Step 3: Decide.

Water is a compound because atoms of different elements are chemically joined in a fixed ratio.

Worked Example 3: Iron and Sulfur Compared with Iron Sulfide

Iron filings and sulfur powder before heating:

• iron is grey and magnetic
• sulfur is yellow and not magnetic
• particles are mixed but not chemically joined
• iron can be separated with a magnet

Iron sulfide after heating:

• black solid forms
• properties are different
• iron cannot be separated with a magnet in the same simple way
• particles are chemically joined in a compound

Conclusion: iron and sulfur before heating are a mixture. Iron sulfide after heating is a compound.

Worked Example 4: Using a Word Equation

magnesium + oxygen -> magnesium oxide

Step 1: Identify reactants.

Magnesium and oxygen are the reactants.

Step 2: Identify product.

Magnesium oxide is the product.

Step 3: Explain the change.

Magnesium atoms and oxygen atoms become chemically joined. A new compound forms. The atoms are rearranged, not created or destroyed.

Diagram Interpretation Questions

Use the diagrams below.

Diagram 1:  A   A   A   A

Diagram 2:  A-A   A-A   A-A

Diagram 3:  A-B   A-B   A-B

Diagram 4:  A-A   B   A-B   B-B

Questions:

1. Which diagram shows an element made of single atoms?
2. Which diagram shows an element made of molecules?
3. Which diagram shows a compound?
4. Which diagram shows a mixture?
5. Explain why Diagram 3 is not a mixture.
6. Explain why Diagram 2 is not a compound.

Model answers:

1. Diagram 1 shows an element made of single atoms because all particles are single atoms of the same type.
2. Diagram 2 shows an element made of molecules because each particle has two atoms joined, but all atoms are type A.
3. Diagram 3 shows a compound because each particle contains different atom types chemically joined in a fixed pattern.
4. Diagram 4 shows a mixture because different particles are present together.
5. Diagram 3 is not a mixture because all particles are identical A-B particles and the different atoms are chemically joined.
6. Diagram 2 is not a compound because it contains only one type of atom.

Exam-Style Questions

Multiple-Choice Questions

Choose the best answer for each question.

1. What is matter? A. Anything that is visible
   B. Anything that has mass and takes up space
   C. Anything that is solid
   D. Anything that is made by humans

2. Which statement about atoms is correct? A. Atoms can be seen with ordinary school microscopes
   B. Atoms are tiny versions of the material they make
   C. Atoms are tiny particles that make up matter
   D. Atoms only exist in solids

3. Which substance is an element? A. Water
   B. Carbon dioxide
   C. Copper
   D. Sea water

4. Which substance is a compound? A. Oxygen gas, O2
   B. Helium, He
   C. Sodium chloride, NaCl
   D. Air

5. Why is air a mixture? A. It contains only oxygen atoms
   B. It contains gases that are chemically joined in one formula
   C. It contains different gases not chemically joined in one fixed ratio
   D. It is invisible

6. What does the formula CO2 show? A. One cobalt atom
   B. One carbon atom and two oxygen atoms
   C. One carbon atom and one oxygen molecule
   D. Two carbon atoms and one oxygen atom

7. Which formula represents a molecule of an element? A. H2O
   B. CO2
   C. O2
   D. NaCl

8. What is the correct symbol for chlorine? A. C
   B. CL
   C. Cl
   D. cl

9. Which part of an atom has a positive charge? A. Proton
   B. Neutron
   C. Electron
   D. Molecule

10. Which method could separate iron filings from sulfur powder before heating? A. Evaporation
    B. A magnet
    C. Freezing
    D. Burning

11. What is a group in the periodic table? A. A row
    B. A column
    C. A mixture
    D. A formula

12. Which statement about compounds is correct? A. Compounds are always easy to separate with a magnet
    B. Compounds contain substances not chemically joined
    C. Compounds contain atoms of different elements chemically joined
    D. Compounds have exactly the same properties as their elements

Fill-in-the-Blank Questions

Use these words: atom, element, compound, mixture, molecule, formula, nucleus, electron, physical, chemical.

1. A tiny particle that makes up matter is called an ________.
2. A pure substance made from one type of atom is an ________.
3. A pure substance made from different elements chemically joined is a ________.
4. Air is a ________ because its gases are not chemically joined in one fixed ratio.
5. A group of atoms chemically joined together is a ________.
6. A chemical ________ shows the types and numbers of atoms in a substance.
7. The central part of an atom is the ________.
8. An ________ has a negative charge.
9. Filtering sand from water is a ________ separation.
10. Burning magnesium is a ________ change because a new substance forms.

Short-Answer Questions

1. Explain why copper is an element.
2. Explain why water is a compound.
3. Explain why salt water is a mixture.
4. What is the difference between Co and CO?
5. Why is O2 a molecule but not a compound?
6. In the formula NH3, which elements are present and how many atoms of each are shown?
7. What evidence might show that a chemical reaction has happened?
8. Why can a magnet separate iron from sulfur before heating but not separate iron from iron sulfide in the same simple way?
9. What is the difference between an observation and a conclusion?
10. Why are particle diagrams useful even though they are not drawn to scale?

Formula Interpretation Questions

Complete the table.

Formula	Elements present	Atom count	Element or compound?
H2
H2O
CO2
CH4
MgO
NaCl

Periodic Table Extract Questions

Use the periodic table extract in this pack.

1. What is the symbol for aluminium?
2. What is the name of the element with symbol S?
3. Which element has atomic number 8?
4. Name two metals in period 3.
5. Name two non-metals in period 2.
6. Which elements shown are in group 18?
7. What simple property do helium, neon and argon share in the table?
8. Describe one pattern shown by group 1 in the extract.

Data and Graph Questions

Use the dry air data table and bar chart.

1. What percentage of dry air is nitrogen?
2. What percentage of dry air is oxygen?
3. Calculate the difference between the nitrogen percentage and oxygen percentage.
4. Which gas is shown as 0.9% of dry air?
5. Carbon dioxide has a small percentage in dry air. Explain why it is still important.
6. Explain why dry air is classified as a mixture.
7. Give one limitation of the bar chart.

Practical Method and Variables Questions

A student wants to compare how well three mixtures can be separated:

• iron filings and sulfur powder
• sand and water
• salt water

1. Name a suitable separation method for each mixture.
2. Identify the independent variable.
3. Identify one dependent variable the student could record.
4. Give two control variables.
5. Explain one safety precaution.
6. Explain how the student could improve reliability.
7. Give one limitation of the investigation.

Longer 6-8 Mark Question

A student says, "Salt water and water are both compounds because they both look the same all the way through."

Explain why the student is wrong. Use the words compound, mixture, chemically joined, fixed ratio and separation in your answer.

Model Answers

Multiple-Choice Answers

1. B. Matter is anything that has mass and takes up space.
2. C. Atoms are tiny particles that make up matter.
3. C. Copper is an element because it contains only copper atoms.
4. C. Sodium chloride is a compound because sodium and chlorine are chemically joined.
5. C. Air contains different gases that are not chemically joined in one fixed ratio.
6. B. CO2 contains one carbon atom and two oxygen atoms.
7. C. O2 is a molecule of an element.
8. C. Chlorine is Cl, with a capital C and lower-case l.
9. A. A proton has a positive charge.
10. B. Iron is magnetic, sulfur is not.
11. B. A group is a column in the periodic table.
12. C. Compounds contain atoms of different elements chemically joined.

Fill-in-the-Blank Answers

1. atom
2. element
3. compound
4. mixture
5. molecule
6. formula
7. nucleus
8. electron
9. physical
10. chemical

Short-Answer Model Answers

1. Copper is an element because it is a pure substance made from only copper atoms.
2. Water is a compound because it contains hydrogen and oxygen atoms chemically joined in a fixed ratio.
3. Salt water is a mixture because salt and water are together but not chemically joined. The water can be evaporated to leave salt behind.
4. Co is the symbol for cobalt. CO means carbon and oxygen together in a formula. The capital letters change the meaning.
5. O2 is a molecule because two oxygen atoms are chemically joined. It is not a compound because it contains only one type of atom.
6. NH3 contains nitrogen and hydrogen. It shows one nitrogen atom and three hydrogen atoms.
7. Evidence might include colour change, gas produced, heat or light given out, or a new solid forming.
8. Before heating, iron and sulfur are a mixture, so iron keeps its magnetic property. In iron sulfide, iron is chemically joined with sulfur in a compound with different properties.
9. An observation is something noticed or measured directly. A conclusion is an explanation based on evidence.
10. Particle diagrams help show types of particles, whether atoms are joined, and whether a substance is an element, compound or mixture, even though they are simplified models.

Formula Interpretation Model Answers

Formula	Elements present	Atom count	Element or compound?
H2	Hydrogen	2 hydrogen atoms	Element
H2O	Hydrogen and oxygen	2 hydrogen, 1 oxygen	Compound
CO2	Carbon and oxygen	1 carbon, 2 oxygen	Compound
CH4	Carbon and hydrogen	1 carbon, 4 hydrogen	Compound
MgO	Magnesium and oxygen	1 magnesium, 1 oxygen	Compound
NaCl	Sodium and chlorine	1 sodium, 1 chlorine in the simplest ratio	Compound

Periodic Table Model Answers

1. Aluminium has the symbol Al.
2. S is sulfur.
3. Oxygen has atomic number 8.
4. Sodium, magnesium and aluminium are metals in period 3; any two are correct.
5. Carbon, nitrogen, oxygen, fluorine and neon are non-metals in period 2; any two are correct.
6. Helium, neon and argon are in group 18 in the extract.
7. They are unreactive gases.
8. Group 1 contains hydrogen, lithium and sodium in this extract. Lithium and sodium are metals, while hydrogen is a non-metal, so the group shows a column pattern but not every property is identical.

Data and Graph Model Answers

1. Nitrogen is about 78% of dry air.
2. Oxygen is about 21% of dry air.
3. The difference is 78 - 21 = 57 percentage points.
4. Argon is shown as 0.9%.
5. Carbon dioxide is important because it is involved in climate change, photosynthesis and breathing out, even though its percentage is small.
6. Dry air is a mixture because it contains several gases that are not chemically joined together in one fixed formula.
7. One limitation is that the chart is approximate and does not include water vapour.

Practical Method and Variables Model Answers

1. Iron filings and sulfur: use a magnet. Sand and water: filtration. Salt water: evaporation.
2. The independent variable is the mixture being tested.
3. A dependent variable could be whether the mixture separates, the mass recovered, or the observation after separation.
4. Control variables could include sample mass, volume of water, type of filter paper, heating time, and apparatus used.
5. Wear eye protection, avoid touching hot equipment, and take care when heating during evaporation.
6. Repeat each separation test and compare results with other groups.
7. A limitation is that separation may be incomplete, so some material may be lost or left behind.

Longer 6-8 Mark Model Answer

The student is wrong because water and salt water are not the same type of substance. Water is a compound because hydrogen atoms and oxygen atoms are chemically joined in a fixed ratio. Each water molecule is H2O, so it contains two hydrogen atoms and one oxygen atom. Salt water is a mixture because sodium chloride and water are together but not chemically joined into one new substance. The amount of salt in salt water can change, so it does not have one fixed ratio like a compound. Salt water may look the same all the way through, but appearance alone is not enough evidence. It can be separated by physical separation, such as evaporation, which removes water and leaves salt behind. This shows salt water is a mixture, while water is a compound.

Extra Practice: Classification Challenge

Classify each example as element, compound or mixture. Give a reason.

Example	Classification	Reason
Copper wire
Oxygen gas, O2
Carbon dioxide, CO2
Sea water
Sodium chloride, NaCl
Soil
Helium gas
Brass
Methane, CH4
Iron filings and sulfur before heating

Model answers:

Example	Classification	Reason
Copper wire	Element	It contains only copper atoms.
Oxygen gas, O2	Element	It contains only oxygen atoms, even though they are joined in pairs.
Carbon dioxide, CO2	Compound	Carbon and oxygen atoms are chemically joined.
Sea water	Mixture	Water, salts and other substances are together but not chemically joined.
Sodium chloride, NaCl	Compound	Sodium and chlorine are chemically joined in a fixed ratio.
Soil	Mixture	It contains different substances such as minerals, organic matter, water and air.
Helium gas	Element	It contains only helium atoms.
Brass	Mixture	It is an alloy, a mixture of metals, mainly copper and zinc.
Methane, CH4	Compound	Carbon and hydrogen atoms are chemically joined.
Iron filings and sulfur before heating	Mixture	Iron and sulfur are together but not chemically joined and can be separated with a magnet.

Revision Checklist

Use this checklist to review the topic.

• I can define matter as anything with mass that takes up space.
• I can explain that all matter is made from atoms.
• I know atoms are too small to see with the naked eye or ordinary school microscopes.
• I can label a simple atom model with nucleus, protons, neutrons and electrons.
• I know protons are positive, neutrons have no charge, and electrons are negative.
• I can define an element as a pure substance made from one type of atom.
• I can recognise chemical symbols such as H, O, C, Fe, Na, Cl, Cu and He.
• I can explain why capital letters matter in chemical symbols.
• I can define a molecule as atoms chemically joined together.
• I can explain why O2 is a molecule but not a compound.
• I can define a compound as atoms of different elements chemically joined in a fixed ratio.
• I can explain why compounds have different properties from the elements they contain.
• I can define a mixture as substances together but not chemically joined.
• I can compare elements, compounds and mixtures.
• I can read simple formulae such as H2O, CO2, CH4, NH3, NaCl and MgO.
• I know that a small number in a formula applies only to the element before it.
• I can classify particle diagrams as elements, compounds or mixtures.
• I can explain the difference between chemical change and physical mixing.
• I can describe evidence that iron and sulfur form iron sulfide after heating.
• I can explain conservation of atoms as atoms being rearranged, not created or destroyed.
• I can read a simple periodic table tile with atomic number, symbol and name.
• I know groups are columns and periods are rows in the periodic table.
• I can describe metals as mostly on the left and centre and non-metals mostly on the right.
• I can give examples of common elements and their uses.
• I can use evidence from tables, diagrams and graphs to classify substances.
• I can identify independent, dependent and control variables in a simple separation investigation.
• I can suggest improvements linked to reliability, repeatability, accuracy or precision.
• I can write a conclusion that uses observations as evidence.

Final Summary

Atoms are the tiny particles that make up all matter. An element contains only one type of atom. A molecule is a group of atoms chemically joined together. A compound contains atoms of different elements chemically joined in a fixed ratio. A mixture contains substances that are together but not chemically joined, so mixtures can often be separated by physical methods.

Chemical symbols and formulae help scientists describe substances clearly. H2O means water contains hydrogen and oxygen, with two hydrogen atoms and one oxygen atom in each molecule. CO2 means carbon dioxide contains one carbon atom and two oxygen atoms. Capital letters matter: Co is cobalt, but CO means carbon and oxygen.

The periodic table arranges elements in patterns. Groups are columns, periods are rows, metals are mostly on the left and centre, and non-metals are mostly on the right. Understanding these patterns helps you classify substances, interpret diagrams, and explain chemical changes using evidence.