Atoms, Elements and Compounds — AQA GCSE Chemistry

SPEC 4.1.1.1 • ATOMIC STRUCTURE

Atoms, Elements & Compounds

The vocabulary of every reaction, every equation, and every structure in GCSE Chemistry. Get these definitions exactly right and the rest of the course gets easier.

Why This Lesson Matters

This is where chemistry starts. Every reaction you'll ever balance, every structure you'll ever draw, every calculation you'll ever do — all of it is built on three precisely-defined ideas: atom, element, and compound.

What's at stake

Definitions in this lesson are mark-scheme verbatim. AQA examiners expect the exact words. "Two or more elements joined together" loses the mark; "two or more elements chemically combined in fixed proportions" earns it.

Topic priority

Foundational. There are no questions in past papers that test this directly as a one-marker — but the language is tested implicitly in every other question. Sloppy here = sloppy everywhere.

Definitions: Atom & Element

Two definitions to memorise word for word. Mark-scheme wording in bold.

Atom

The smallest part of an element that can exist.

Element

A substance made of only one type of atom.

The link

An element is the bulk substance — e.g. a piece of copper. An atom is the smallest individual unit of that substance. There are about 100 different elements, all shown in the periodic table.

Definition: Compound

This is the one students get wrong most often. The phrase that earns the mark is "chemically combined in fixed proportions".

Compound

A substance formed when two or more elements are chemically combined in fixed proportions.

Why those exact words?

"Chemically combined" means a chemical reaction has occurred — one or more new substances are formed, often with a detectable energy change. You cannot separate the elements back by physical means (filtering, evaporating, etc.) — only by another chemical reaction.

"Fixed proportions" means the ratio of elements is always the same. Water is always 2 hydrogen : 1 oxygen — that's what makes it H₂O. Change the proportions and you get an entirely different compound: H₂O₂ (hydrogen peroxide) has the same elements in a 1:1 ratio and is a different substance with different properties.

The contrast — coming up

If two substances are mixed without bonding, that's a mixture — not a compound. We'll see the visual difference next.

The Visual Difference

Three particle diagrams. Same atoms — completely different substances. The labels matter as much as the picture.

The decisive test

Are the atoms chemically bonded? Yes → compound. No → mixture. That single question separates the two for every question you'll meet.

Chemical Symbols

Every element has a unique one- or two-letter symbol. There is one rule, and AQA enforces it strictly.

The one rule

First letter is always a CAPITAL. If there's a second letter, it is always lower case.

Sodium = Na ✓ | NA ✘ | na ✘

You must know the symbols for the first 20 elements, all Group 1, all Group 7, and any element named in the spec. A small starter set:

H

hydrogen

C

carbon

N

nitrogen

O

oxygen

Na

sodium

Mg

magnesium

Al

aluminium

Cl

chlorine

Ca

calcium

Fe

iron

Examiner note

Examiners report losing marks for "badly written symbols" — particularly Zn written as ZN, or Cl written as CL. Treat symbol case as carefully as you'd treat the digits in a calculation.

Naming a compound from a formula

The spec also requires you to name compounds from given formulae. Read the symbols in order, swap the second non-metal ending to "-ide" if there are only two elements:

MgO → magnesium oxide

NaCl → sodium chloride

CaBr₂ → calcium bromide

If oxygen is in a polyatomic group (e.g. SO₄, NO₃, CO₃), the ending stays "-ate" or "-ite": Na₂SO₄ = sodium sulfate, CaCO₃ = calcium carbonate.

Molecule & Formula

Two more terms the spec defines explicitly. Both have a precise meaning that's tested when you meet structures later in the course.

Molecule

A particle containing two or more atoms chemically bonded together. Can be the same element (O₂, Cl₂) or different elements (H₂O, CO₂).

Formula

A representation using chemical symbols to show the ratio of atoms in a substance.

Reading a formula

In H₂SO₄: 2 hydrogen atoms, 1 sulfur atom, 4 oxygen atoms. The small number (subscript) tells you how many of the atom immediately to its left. No subscript = 1 atom.

Coming later — important caveat

Not every compound is made of molecules. Giant ionic lattices (NaCl) and giant covalent structures (diamond, SiO₂) are not molecular. Calling them "molecules" loses marks in Topic 2.

Compound vs Mixture — The Trap

A very common 2-mark question: "Explain why X is a compound and not a mixture." Same content, two answers — only one scores.

Weak student

"Copper sulfate is a compound because it has copper and sulfur and oxygen all together."

Why it fails: describes the contents but not the bonding. A mixture also has multiple elements together. Could be 0/2.

Strong student

"Copper sulfate is a compound because copper, sulfur and oxygen are chemically combined in fixed proportions, so it can only be separated back into elements by a chemical reaction."

Why it scores: hits the spec phrase ("chemically combined", "fixed proportions") and contrasts with a mixture (which can be physically separated).

The two-step test

Step 1: Are the elements chemically combined? Step 2: Is the ratio fixed? Yes to both → compound. Otherwise → mixture.

Word vs Symbol Equations

Reactions are written two ways at GCSE. Foundation often asks for the word equation; both tiers can be asked for the balanced symbol equation.

Word equation

copper carbonate → copper oxide + carbon dioxide

Names only. No symbols, no numbers. Useful when you don't need to count atoms — but it can't be balanced because there are no formulae.

Balanced symbol equation

CuCO₃ → CuO + CO₂

Each element must have the same total number of atoms on the left and right. Left: 1 Cu, 1 C, 3 O. Right: 1 Cu, 1 C, 1+2 = 3 O. Balanced.

Mark-scheme rule

For "balanced symbol equation" questions: 1 mark for correct formulae; 1 mark for balancing. Wrong formula = both marks lost, even if the equation "balances" with the wrong substance.

Balancing Rules

Two types of number live in chemical equations. You are only ever allowed to change one of them.

TypeWhereCan you change it?Why

CoefficientBig number in front of the formula. e.g. 2 H₂OYES — to balanceTells you how many of that whole formula unit are present.

SubscriptSmall number inside the formula. e.g. H₂ONEVERTells you the identity of the substance. Change it = different compound.

The trap students fall into

To balance more oxygens on the right, students change H₂O to H₂O₂. But:

H₂O = water | H₂O₂ = hydrogen peroxide

Two completely different substances. The "balanced" equation now describes a different reaction. Mark lost.

The rule, one line

Change coefficients to balance. Never change subscripts.

HT only — coming later

Higher Tier students must also be able to write balanced ionic equations and half equations. These show only the species that change, with charges balanced on both sides. Both are taught in detail in Topic 4 (electrolysis) and where redox reactions appear later in the course — not required at this stage.

Worked Example 2 MARKS01

Copper carbonate reacts with sulfuric acid to form copper sulfate, water, and carbon dioxide. Write the balanced symbol equation.

1

Write correct formulae for every reactant and product.

copper carbonate = CuCO₃ | sulfuric acid = H₂SO₄ | copper sulfate = CuSO₄ | water = H₂O | carbon dioxide = CO₂

2

Lay it out and count atoms each side.CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂ Left: Cu = 1, C = 1, O = 3+4 = 7, H = 2, S = 1

Right: Cu = 1, C = 1, O = 4+1+2 = 7, H = 2, S = 1

3

Check. Every element balances at 1:1 — no coefficients needed.

Final answer:CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂

Mark scheme: 1 mark for correct formulae, 1 mark for balancing. Both achievable here in one step.

PEE Framework — Worked Chain

Use this structure on every "explain why" question. The mark scheme rewards a chain that goes from a precise point, through specific evidence, to a mechanism-based explanation.

Q: Explain why copper sulfate is a compound and not a mixture. 2 MARKS

P — PointCopper sulfate is a compound because copper, sulfur and oxygen are chemically combined in fixed proportions.

E — EvidenceIts formula is CuSO₄ — a defining ratio of 1 Cu : 1 S : 4 O in every sample.

E — ExplainIt can only be separated back into elements by a chemical reaction — not by physical means like filtration.

How marks are scored

Mark 1: identifying chemical combination + fixed proportions. Mark 2: linking to inability of physical separation. The contrast with a mixture is what unlocks the second mark.

Examiner Alert

P1 Jun 2024 Q01.1 — direct quote pattern

On the copper carbonate + sulfuric acid question, examiners reported that around 1 in 3 students named the salt incorrectly, writing "copper chloride" or "copper sulfide" instead of copper sulfate. Some also wrote that hydrogen was produced — it isn't.

The two rules to memorise

Salt naming: the salt's name = the metal from the carbonate (or oxide) + the anion of the acid:

sulfuric acid → sulfate (–ate)

hydrochloric acid → chloride (–ide)

nitric acid → nitrate (–ate)

Gas product test:

acid + carbonate → salt + water + carbon dioxide

acid + metal → salt + hydrogen

The trap, one line

Carbonate ≠ metal. Carbonates give CO₂; metals give H₂.

Common Errors

Students often writeCorrect versionWhy it matters

"Copper chloride" (when sulfuric acid is the reactant)Copper sulfateAnion of the salt comes from the acid used, not a different acid

Acid + carbonate → salt + hydrogenAcid + carbonate → salt + water + CO₂Hydrogen is produced from acid + metal , not acid + carbonate

Changing H₂O to H₂O₂ "to balance"Add a coefficient: 2H₂OSubscripts are part of the substance's identity

Lower-case symbols: na , clCorrect case: Na , ClExaminer reports flag "badly written symbols" as marks lost

"Copper sulfate is a compound because it has different elements""...because the elements are chemically combined in fixed proportions "Mixtures also contain different elements — fails to distinguish

Practice Question 1

Magnesium carbonate reacts with hydrochloric acid. Name the salt produced and the gas produced.2 MARKS

(Pattern from AQA 8462 Paper 1 Q01.1, June 2024)

1

Identify the metal of the carbonate. Magnesium carbonate → metal is magnesium.

2

Identify the anion from the acid. Hydrochloric acid → chloride (–ide ending).

3

Combine for the salt name. Magnesium + chloride = magnesium chloride.

4

Gas test. Acid + carbonate → CO₂. Gas = carbon dioxide.

Final answer: Salt = magnesium chloride. Gas = carbon dioxide. 1 + 1

Practice Question 2

Write the balanced symbol equation for the reaction:2 MARKS

magnesium + oxygen → magnesium oxide

1

Write correct formulae. Magnesium = Mg. Oxygen gas = O₂ (a diatomic molecule). Magnesium oxide = MgO.

2

Initial unbalanced.Mg + O₂ → MgO. Count: left has 2 O, right has 1 O. Not balanced.

3

Balance with coefficients only. Put 2 in front of MgO: Mg + O₂ → 2MgO. Now Mg: 1 left, 2 right — also off.

4

Fix Mg. Put 2 in front of Mg: 2Mg + O₂ → 2MgO. Now Mg: 2 = 2, O: 2 = 2. ✓

Final answer:2Mg + O₂ → 2MgO

Marks: 1 for correct formulae (especially O₂ not O), 1 for balanced equation.

Practice Question 3

Air is a mixture. Water is a compound. Explain the difference between a mixture and a compound, using these two examples.3 MARKS

P

Point. A compound has elements chemically combined in fixed proportions; a mixture has substances together but not chemically combined.

E

Evidence. Water has a defining formula: H₂O — exactly 2 hydrogen atoms to 1 oxygen atom in every sample. Air has no defining formula — it contains separate molecules of N₂, O₂, Ar and CO₂ that are not bonded to each other.

E

Explain. Water can only be separated back to hydrogen and oxygen by a chemical reaction (e.g. electrolysis). Air can be separated by a physical process — fractional distillation of liquid air — because the gases are not chemically bonded together.

Mark allocation: 1 mark for the chemical-combination distinction, 1 mark for the defining-formula contrast (H₂O), 1 mark for the physical-vs-chemical separation point.

Summary

Mark-Scheme Definitions

Atom: the smallest part of an element that can exist

Element: a substance made of only one type of atom

Compound: two or more elements chemically combined in fixed proportions

Molecule: two or more atoms chemically bonded together

The Compound vs Mixture Test

Are the elements chemically combined? Yes → compound, fixed ratio, only chemical separation. No → mixture, any ratio, physical separation works.

Equation Rules

Change coefficients (in front) to balance

Never change subscripts — they define the substance

Acid + carbonate → salt + water + CO₂

Acid + metal → salt + H₂

Salt anion comes from the acid: sulfuric → sulfate, hydrochloric → chloride, nitric → nitrate

Exam Tip — the most-missed mark on this topic

When asked to distinguish compound from mixture, always state both "chemically combined" and "fixed proportions". Saying just one of them caps you at 1 mark out of 2.

Up Next — Lesson 1.02: Mixtures

The five physical separation techniques on the spec — filtration, crystallisation, simple distillation, fractional distillation, and chromatography. Plus how to choose the right one for the right mixture.