Atoms, Elements and Compounds

This lesson covers the fundamental concepts of atoms, elements and compounds as required by the AQA GCSE Chemistry specification (5.1.1). All matter in the universe is made up of atoms. Understanding what atoms are, how elements differ from compounds, and how chemical formulae represent substances is essential for every topic you will study in chemistry.

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What Is an Atom?

An atom is the smallest particle of an element that can take part in a chemical reaction. Atoms are incredibly small — typically around 0.1 to 0.3 nanometres in diameter (1 nm = 1 x 10^-9 m). Everything around you — solids, liquids, gases, and even your own body — is made up of atoms.

Key facts about atoms:

• Atoms are the building blocks of all matter.
• There are approximately 118 known elements, each made up of a single type of atom.
• Atoms cannot be broken down into simpler substances by chemical reactions — only by nuclear reactions.
• Each element has its own unique type of atom, characterised by the number of protons in its nucleus.

Exam Tip: The AQA specification states that all substances are made of atoms. When defining an atom in the exam, always say it is the "smallest part of an element that can exist as a stable entity." Avoid saying atoms are indivisible — they contain sub-atomic particles.

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Elements

An element is a substance that contains only one type of atom. Elements cannot be broken down into simpler substances by chemical means. Each element is represented by a chemical symbol from the periodic table.

Element	Symbol	Type
Hydrogen	H	Non-metal
Oxygen	O	Non-metal
Carbon	C	Non-metal
Iron	Fe	Metal
Sodium	Na	Metal
Chlorine	Cl	Non-metal
Gold	Au	Metal
Copper	Cu	Metal

Rules for Chemical Symbols

• The first letter is always a capital letter.
• If there is a second letter, it is always lower case.
• Some symbols come from Latin names — for example, Fe from ferrum (iron), Na from natrium (sodium), Au from aurum (gold).

Exam Tip: Always write chemical symbols correctly. "CO" means carbon monoxide (a compound), but "Co" means cobalt (an element). Case matters enormously in chemistry.

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Compounds

A compound is a substance that contains two or more different elements that are chemically bonded together. The atoms in a compound are held together by chemical bonds — either ionic bonds or covalent bonds. Compounds can only be separated into their elements by chemical reactions, not by physical methods.

Compound	Formula	Elements Present
Water	H2O	Hydrogen, Oxygen
Carbon dioxide	CO2	Carbon, Oxygen
Sodium chloride (table salt)	NaCl	Sodium, Chlorine
Magnesium oxide	MgO	Magnesium, Oxygen
Iron sulfide	FeS	Iron, Sulfur
Calcium carbonate (limestone)	CaCO3	Calcium, Carbon, Oxygen

Properties of Compounds

Compounds have properties that are different from the elements they contain. For example:

• Sodium is a highly reactive metal that explodes on contact with water.
• Chlorine is a poisonous green gas.
• Sodium chloride (table salt) is a harmless white solid used to flavour food.

This shows that when elements combine to form a compound, the properties change completely because the atoms are bonded together in a new arrangement.

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Chemical Formulae

A chemical formula shows the number and type of atoms in a molecule or compound. The small number written after an element's symbol (a subscript) tells you how many atoms of that element are present.

Formula	Meaning
H2	2 hydrogen atoms bonded together (a molecule of hydrogen)
O2	2 oxygen atoms bonded together (a molecule of oxygen)
H2O	2 hydrogen atoms and 1 oxygen atom bonded together
CO2	1 carbon atom and 2 oxygen atoms bonded together
H2SO4	2 hydrogen atoms, 1 sulfur atom, and 4 oxygen atoms
Ca(OH)2	1 calcium atom, 2 oxygen atoms, and 2 hydrogen atoms

Working with Brackets in Formulae

When a formula contains brackets, the subscript outside the bracket multiplies everything inside. For example:

• Ca(OH)2 means 1 Ca, 2 O, and 2 H (the 2 multiplies both O and H).
• Mg(NO3)2 means 1 Mg, 2 N, and 6 O (the 2 multiplies both N and the 3 oxygens).

Exam Tip: When counting atoms in a formula with brackets, always multiply the subscript outside by each element inside the bracket. A common mistake is to forget to multiply — for example, in Mg(NO3)2, there are 6 oxygen atoms (3 x 2), not 3.

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Atoms, Elements, Compounds and Mixtures — Overview

graph TD
    A["Matter"] --> B["Pure Substances"]
    A --> C["Mixtures"]
    B --> D["Elements"]
    B --> E["Compounds"]
    D --> F["One type of atom only"]
    E --> G["Two or more different atoms chemically bonded"]
    C --> H["Two or more substances NOT chemically bonded"]

    style A fill:#2c3e50,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#8e44ad,color:#fff

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Molecules

A molecule is formed when two or more atoms are covalently bonded together. Molecules can be:

• Elements — if the atoms are all the same type, e.g. O2 (oxygen gas), N2 (nitrogen gas), H2 (hydrogen gas).
• Compounds — if the atoms are different types, e.g. H2O (water), CO2 (carbon dioxide).

Some elements exist naturally as molecules rather than individual atoms. The diatomic elements (elements that exist as pairs of atoms) are:

Element	Formula
Hydrogen	H2
Nitrogen	N2
Oxygen	O2
Fluorine	F2
Chlorine	Cl2
Bromine	Br2
Iodine	I2

Exam Tip: Remember the diatomic elements with the mnemonic: Have No Fear Of Ice Cold Beer (H2, N2, F2, O2, I2, Cl2, Br2). In equations, always write these elements as diatomic molecules — writing just "O" instead of "O2" will cost you marks.

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Chemical Reactions and Conservation of Atoms

In a chemical reaction, atoms are rearranged to form new substances. The key principle is that atoms are never created or destroyed during a chemical reaction — they are simply rearranged into different combinations. This means:

• The total number of atoms of each element is the same on both sides of a balanced equation.
• The total mass of the reactants equals the total mass of the products (conservation of mass).

For example, when iron reacts with sulfur:

Iron + Sulfur → Iron sulfide

Fe + S → FeS

The iron atoms and sulfur atoms are rearranged, but no atoms are lost or gained.

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Common Exam Mistakes

• Confusing elements and compounds — an element has only one type of atom.
• Writing chemical symbols incorrectly — e.g. writing "co" instead of "Co" for cobalt.
• Forgetting that some elements are diatomic — always write O2, H2, N2, etc.
• Confusing mixtures and compounds — in a compound, atoms are chemically bonded; in a mixture, substances are just mixed together.
• Saying atoms are "destroyed" or "created" in a chemical reaction — they are rearranged.

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Summary

• An atom is the smallest particle of an element that can take part in a chemical reaction.
• An element contains only one type of atom and cannot be broken down by chemical means.
• A compound contains two or more different elements chemically bonded together.
• Chemical formulae show the number and type of atoms in a substance.
• Subscript numbers tell you how many atoms of each element are present.
• Compounds have different properties from the elements they contain.
• In chemical reactions, atoms are rearranged but never created or destroyed.
• Some elements exist as diatomic molecules (H2, N2, O2, F2, Cl2, Br2, I2).

Exam Tip: A 6-mark question may ask you to explain the difference between elements, compounds and mixtures. Use specific examples, include chemical formulae, and explain that compounds are chemically bonded while mixtures are not. Always mention that compounds have different properties from their constituent elements.

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Worked Example — Counting Atoms in a Formula

Question: How many atoms of each element are in one formula unit of ammonium sulfate, (NH4)2SO4? State the total number of atoms.

Worked solution:

1. Inside the bracket (NH4) there is 1 nitrogen and 4 hydrogen. The subscript 2 outside multiplies both.
2. Nitrogen atoms = 1 x 2 = 2.
3. Hydrogen atoms = 4 x 2 = 8.
4. Sulfur atoms = 1 (from SO4).
5. Oxygen atoms = 4 (from SO4).
6. Total atoms per formula unit = 2 + 8 + 1 + 4 = 15 atoms.

Common mistake: Students forget to multiply every element inside the bracket by the subscript outside. Writing "1 N, 4 H" for (NH4)2 misses the multiplier and throws off every subsequent balancing calculation.

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Worked Example — Balancing a Simple Equation

Balance: Mg + O2 → MgO

1. Count atoms on each side. Left: 1 Mg, 2 O. Right: 1 Mg, 1 O.
2. Oxygen is unbalanced. Place a 2 in front of MgO: Mg + O2 → 2MgO.
3. Now right side has 2 Mg. Place a 2 in front of Mg on the left: 2Mg + O2 → 2MgO.
4. Final check: Left — 2 Mg, 2 O. Right — 2 Mg, 2 O. Balanced.

Common mistake: Changing subscripts inside a formula to balance an equation. You can only change the big numbers (coefficients) in front of a formula. Writing Mg + O → MgO is wrong because oxygen is diatomic (O2).

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Comparison — Elements, Compounds and Mixtures

Feature	Element	Compound	Mixture
Types of atom	One only	Two or more, chemically bonded	Two or more, not bonded
Separation	Nuclear reactions only	Chemical reaction required	Physical methods (filtration, distillation)
Composition	Fixed (one type)	Fixed ratio (e.g. H2O is always 2:1)	Variable
Properties	Own unique properties	New properties, different from the elements	Each component retains its properties
Example	Oxygen (O2), Iron (Fe)	Water (H2O), Sodium chloride (NaCl)	Air, sea water, brass

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Percentage Composition by Mass

For a compound you can calculate the percentage by mass of each element using relative atomic masses from the periodic table.

Worked example — percentage of oxygen in water (H2O):

• Mr of H2O = (2 x 1) + 16 = 18
• Mass of O in one formula unit = 16
• Percentage of O = (16 / 18) x 100 = 88.9% (to 1 d.p.)
• Percentage of H = (2 / 18) x 100 = 11.1%

Worked example — percentage of nitrogen in ammonium nitrate (NH4NO3):

• Mr = 14 + (4 x 1) + 14 + (3 x 16) = 80
• Total mass of N = 2 x 14 = 28
• Percentage of N = (28 / 80) x 100 = 35.0%

This type of calculation links back to the atom-counting rules and is worth practising repeatedly.

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Answering at different grade levels

Exam-style question (4 marks): Explain the difference between a compound and a mixture, using sodium chloride and a sand-and-salt mixture as examples.

Grade 4–5 answer: A compound is when two elements are joined together. Sodium chloride is made from sodium and chlorine joined together, so it is a compound. A mixture is when two things are mixed but not joined. Sand and salt mixed is a mixture because you can separate them using water and a filter.

Grade 8–9 answer: In a compound, atoms of different elements are chemically bonded in a fixed ratio. Sodium chloride (NaCl) contains Na+ and Cl- ions held by strong ionic bonds in a 1:1 ratio, and its chemical properties (white, soluble solid, melting point 801 degrees C) are entirely different from those of sodium metal or chlorine gas. A mixture contains two or more substances that are not chemically bonded, so each component retains its own properties and the proportions can vary. A sand-and-salt mixture can therefore be separated using purely physical techniques: dissolving the salt in water, filtering to remove the insoluble sand (the residue), and then crystallising the sodium chloride from the filtrate. No chemical reaction is needed, which confirms that the components were not bonded. This link between bonding, fixed ratio and the need for chemical reactions to break compounds apart is what defines a compound against a mixture.

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AQA alignment: This content is aligned with AQA GCSE Chemistry (8462) specification section 5.1 Atomic structure and the periodic table — specifically 5.1.1.1 Atoms, elements and compounds. Assessed on Paper 1.