Atoms, Elements and Compounds

This lesson covers the fundamental building blocks of chemistry — atoms, elements and compounds — as required by the AQA GCSE Combined Science Trilogy specification (8464, Chemistry section 4.1.1). Understanding these concepts is the foundation for everything you will study in chemistry and science more broadly.

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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 — a typical atomic radius is about 1 × 10⁻¹⁰ m (0.1 nanometres).

Key facts about atoms:

• Atoms are the building blocks of all matter — every solid, liquid and gas is made of atoms.
• Each element has its own unique type of atom, characterised by the number of protons in its nucleus.
• There are approximately 118 known elements, each with a different type of atom.
• Atoms cannot be broken down into simpler substances by chemical reactions, although they contain sub-atomic particles (protons, neutrons and electrons).

Exam Tip (AQA 8464): When defining an atom in the exam, say it is "the smallest part of an element that can take part in a chemical reaction." Avoid saying atoms are indivisible — the AQA mark scheme requires acknowledgement that atoms 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 one- or two-letter 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 derive from Latin names — for example, Fe from ferrum (iron), Na from natrium (sodium), Au from aurum (gold).

Exam Tip: Case matters! "CO" is carbon monoxide (a compound), while "Co" is cobalt (an element). Always write symbols carefully.

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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	$\text{H}_2\text{O}$H2​O	Hydrogen, Oxygen
Carbon dioxide	$\text{CO}_2$CO2​	Carbon, Oxygen
Sodium chloride	$\text{NaCl}$NaCl	Sodium, Chlorine
Magnesium oxide	$\text{MgO}$MgO	Magnesium, Oxygen
Iron sulfide	$\text{FeS}$FeS	Iron, Sulfur
Calcium carbonate	$\text{CaCO}_3$CaCO3​	Calcium, Carbon, Oxygen

Properties of Compounds vs Elements

Compounds have properties that are completely different from the elements they contain:

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

This demonstrates that when elements chemically combine, the product has entirely new properties.

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Chemical Formulae

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

Formula	Meaning
$\text{H}_2$H2​	2 hydrogen atoms bonded together
$\text{O}_2$O2​	2 oxygen atoms bonded together
$\text{H}_2\text{O}$H2​O	2 hydrogen atoms and 1 oxygen atom
$\text{CO}_2$CO2​	1 carbon atom and 2 oxygen atoms
$\text{H}_2\text{SO}_4$H2​SO4​	2 H, 1 S and 4 O atoms
$\text{Ca(OH)}_2$Ca(OH)2​	1 Ca, 2 O and 2 H atoms

Working with Brackets

When a formula contains brackets, the subscript outside multiplies everything inside:

• $\text{Ca(OH)}_2$Ca(OH)2​ — 1 Ca, 2 × O = 2 O, 2 × H = 2 H.
• $\text{Mg(NO}_3\text{)}_2$Mg(NO3​)2​ — 1 Mg, 2 × N = 2 N, 2 × 3 O = 6 O.

Exam Tip: A very common mistake is forgetting to multiply — in $\text{Mg(NO}_3\text{)}_2$Mg(NO3​)2​ there are 6 oxygen atoms (3 × 2), not 3.

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Classifying Matter — Overview

graph TD
    A["Matter"] --> B["Pure Substances"]
    A --> C["Mixtures"]
    B --> D["Elements<br/>One type of atom only"]
    B --> E["Compounds<br/>Two or more elements chemically bonded"]
    C --> F["Not chemically bonded<br/>Can be separated by physical methods"]

    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
    style F fill:#d35400,color:#fff

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Diatomic Molecules

Some elements exist naturally as diatomic molecules — pairs of atoms bonded together:

Element	Formula
Hydrogen	$\text{H}_2$H2​
Nitrogen	$\text{N}_2$N2​
Oxygen	$\text{O}_2$O2​
Fluorine	$\text{F}_2$F2​
Chlorine	$\text{Cl}_2$Cl2​
Bromine	$\text{Br}_2$Br2​
Iodine	$\text{I}_2$I2​

Exam Tip: Remember them with: Have No Fear Of Ice Cold Beer. In balanced equations, always write these elements as diatomic molecules — writing "O" instead of "$\text{O}_2$O2​" will cost marks.

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Worked Example

Question: Ammonium nitrate has the formula $\text{NH}_4\text{NO}_3$NH4​NO3​. How many atoms of each element are present in one formula unit?

Answer:

• Nitrogen: 1 (from $\text{NH}_4$NH4​) + 1 (from $\text{NO}_3$NO3​) = 2 N
• Hydrogen: 4 H
• Oxygen: 3 O
• Total atoms = 2 + 4 + 3 = 9 atoms

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Common Mistakes

Mistake	Correction
Calling $\text{O}_2$O2​ a compound	$\text{O}_2$O2​ contains only one type of atom — it is an element
Writing "co" for cobalt	The correct symbol is "Co" — case matters
Saying atoms are "created" or "destroyed" in reactions	Atoms are rearranged in chemical reactions
Confusing mixtures and compounds	In a compound, atoms are chemically bonded; in a mixture, substances are just mixed together

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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.
• A compound contains two or more different elements chemically bonded together.
• Chemical formulae show the number and type of atoms in a substance.
• Compounds have different properties from the elements they contain.
• In chemical reactions, atoms are rearranged but never created or destroyed.
• Seven elements exist as diatomic molecules: $\text{H}_2$H2​, $\text{N}_2$N2​, $\text{O}_2$O2​, $\text{F}_2$F2​, $\text{Cl}_2$Cl2​, $\text{Br}_2$Br2​, $\text{I}_2$I2​.

Exam Tip (AQA 8464): A 6-mark question may ask you to explain the differences between elements, compounds and mixtures. Use specific examples with formulae, explain that compounds are chemically bonded while mixtures are not, and note that compounds have different properties from their constituent elements.

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Extended Worked Examples

Worked Example 1 — Counting atoms in a complex formula

Question: How many atoms of each element are in one formula unit of aluminium sulfate, $\text{Al}_2(\text{SO}_4)_3$Al2​(SO4​)3​? What is the total number of atoms?

Answer:

• Aluminium: 2 atoms (the subscript 2 applies to Al).
• Sulfur: the subscript outside the bracket is 3, so there are 3 × 1 = 3 sulfur atoms.
• Oxygen: inside the bracket we have 4 oxygens, and the subscript 3 outside multiplies everything inside: 3 × 4 = 12 oxygen atoms.
• Total: 2 + 3 + 12 = 17 atoms.

Worked Example 2 — Identifying element vs compound

Question: Classify each of the following as an element, compound or mixture: (a) $\text{O}_3$O3​ (ozone), (b) $\text{H}_2\text{SO}_4$H2​SO4​ (sulfuric acid), (c) air, (d) brass.

Answer:

• (a) $\text{O}_3$O3​ — element (only oxygen atoms, even though three are bonded).
• (b) $\text{H}_2\text{SO}_4$H2​SO4​ — compound (three different elements chemically bonded).
• (c) Air — mixture (nitrogen, oxygen, argon, carbon dioxide — not chemically bonded).
• (d) Brass — mixture (an alloy of copper and zinc; metals are not chemically bonded in an alloy).

Common-Mistake Callout: Learners frequently call $\text{O}_2$O2​ or $\text{O}_3$O3​ a "compound" because there is more than one atom. A compound requires different elements. $\text{O}_2$O2​ and $\text{O}_3$O3​ are allotropes of the element oxygen — pure forms of the same element arranged differently.

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Comparison Table — Elements, Compounds and Mixtures at a Glance

Feature	Element	Compound	Mixture
Types of atom	One	Two or more different	Two or more
Chemical bonding	Atoms of the same kind only	Atoms bonded in fixed ratio	Not bonded together
Fixed composition	Yes (one element)	Yes (fixed formula)	No (variable proportions)
Separation	Cannot be broken down chemically	Only by chemical reaction	By physical methods
Example	Iron (Fe)	Iron(II) sulfide (FeS)	Iron filings + sulfur powder
Properties	Specific to the element	New properties, different from its elements	Mixture shows properties of each component

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Iron + Sulfur — A Classic Demonstration

The reaction between iron filings and sulfur is a classic way to demonstrate the difference between a mixture and a compound.

1. Start with a mixture of iron filings and sulfur powder. The iron is magnetic (can be separated with a magnet), and each component retains its own colour and properties.
2. Heat the mixture strongly in a test tube. The iron and sulfur react chemically to form a new substance — iron(II) sulfide: $\text{Fe}(s) + \text{S}(s) \rightarrow \text{FeS}(s)$Fe(s)+S(s)→FeS(s).
3. The product is a dark grey/black solid that cannot be separated by a magnet, and the sulfur can no longer be dissolved out with carbon disulfide. A new compound has formed, with new properties.

Exam Tip: The iron + sulfur experiment is a favourite of AQA examiners. Be ready to state the observations (red glow spreading through the mixture, even after heating stops — showing an exothermic reaction) and to explain that a new substance has been made with new properties.

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Reasoning About Conservation of Mass

Because atoms are neither created nor destroyed in chemical reactions (they are only rearranged), the total mass of the reactants must equal the total mass of the products. This is the law of conservation of mass.

Example: If 2 g of hydrogen reacts completely with 16 g of oxygen to form water, the mass of water produced must be exactly 18 g. No mass is lost; every hydrogen and oxygen atom is present in the product.

This principle is the basis for balanced chemical equations — the number of atoms of each element must be the same on both sides of the arrow.

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Modelling Particles — Why the Model Matters

Scientists use particle diagrams to represent elements, compounds and mixtures. Different coloured circles represent different elements; touching circles represent chemically bonded atoms.

graph LR
    A["Particle diagrams"] --> B["Element<br/>One kind of circle only"]
    A --> C["Compound<br/>Different circles joined together"]
    A --> D["Mixture<br/>Different circles not joined"]

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

In the exam you might be given a diagram and asked which sample is an element, compound or mixture. Look for: one type of circle, joined or unjoined (element — joined could be $\text{O}_2$O2​); different circles joined together (compound); different circles not joined (mixture of elements or compounds).

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Answering at different grade levels

Grade 3–4 answers typically use everyday language: "An atom is a tiny particle. An element has one kind. A compound has two or more." They often miss the chemical-bonding language.

Grade 5–6 answers use the correct terms: atomic number, mass number, chemical bond and fixed ratio. They state that compounds have new properties different from their elements and can quote a formula such as $\text{H}_2\text{O}$H2​O as "two hydrogen atoms chemically bonded to one oxygen atom."

Grade 7–9 answers integrate deeper reasoning. They explain that the atomic number determines which element an atom is; they recognise that two atoms with the same atomic number but different mass numbers are isotopes (the same element, not different ones); and they know that elements such as chlorine have a non-whole relative atomic mass (35.5) because it is a weighted mean of isotopes. A Grade 7–9 candidate can argue — using electronic configuration — why sodium and chlorine form a compound that is nothing like the elements it came from: sodium loses its outer electron and chlorine gains one, producing the stable ionic compound sodium chloride.

AQA alignment: This content is aligned with AQA GCSE Combined Science: Trilogy (8464) specification section 5.1 Atomic structure and the periodic table — specifically 5.1.1 A simple model of the atom. Assessed on Chemistry Paper 1.