Atoms, Elements and Compounds

This lesson covers the fundamental building blocks of chemistry — atoms, elements and compounds — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand what each of these terms means, recognise chemical symbols and formulae, and begin to use the periodic table to identify elements.

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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 nm (1 × 10⁻¹⁰ m) in radius.

Key facts about atoms:

• Atoms are the building blocks of all matter.
• Each element has its own unique type of atom, identified 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 means, although they do contain smaller particles (protons, neutrons and electrons).

Exam Tip: When defining an atom, say it is "the smallest part of an element that can take part in a chemical reaction." Do not say atoms are indivisible — they contain sub-atomic particles.

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Elements

An element is a substance made of only one type of atom. Elements cannot be broken down into simpler substances by chemical methods. Each element has a unique chemical symbol found on the periodic table.

Chemical Symbols

Each element is represented by either one or two letters:

Element	Symbol	Notes
Hydrogen	H	One letter
Helium	He	Two letters
Carbon	C	One letter
Nitrogen	N	One letter
Oxygen	O	One letter
Sodium	Na	From Latin natrium
Magnesium	Mg	Two letters
Chlorine	Cl	Two letters
Iron	Fe	From Latin ferrum
Copper	Cu	From Latin cuprum
Gold	Au	From Latin aurum
Silver	Ag	From Latin argentum

Rules for Chemical Symbols

1. The first letter is always a capital letter.
2. If there is a second letter, it is always lower case.
3. Some symbols are derived from Latin names — for example, Na from natrium (sodium) and Fe from ferrum (iron).

Exam Tip: Be careful with capitalisation. "CO" means carbon monoxide (a compound), but "Co" means cobalt (an element). Getting this wrong will cost marks.

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The Periodic Table — An Introduction

The periodic table is a chart that organises all known elements in order of atomic number (the number of protons). It was first developed in its modern form by scientists including Dmitri Mendeleev and has been refined as new elements were discovered and our understanding of atomic structure improved.

Key Features of the Periodic Table

Feature	Meaning
Groups (columns)	Elements in the same group have the same number of outer-shell electrons and similar chemical properties
Periods (rows)	Elements in the same period have the same number of electron shells
Metals	Found on the left and centre of the table
Non-metals	Found on the right of the table

You will explore the periodic table in much more detail in later lessons. For now, the key idea is that it is a systematic way of organising elements so that patterns in their behaviour can be identified.

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Compounds

A compound is a substance formed when two or more different elements are chemically bonded together in fixed proportions.

Common Compounds

Compound	Formula	Elements Present
Water	H₂O	Hydrogen, Oxygen
Carbon dioxide	CO₂	Carbon, Oxygen
Sodium chloride	NaCl	Sodium, Chlorine
Magnesium oxide	MgO	Magnesium, Oxygen
Calcium carbonate	CaCO₃	Calcium, Carbon, Oxygen

Key Properties of Compounds

• Compounds have different properties from the elements they contain. For example, sodium is a reactive metal and chlorine is a toxic gas, but sodium chloride is harmless table salt.
• The atoms in a compound are held together by chemical bonds — either ionic bonds (between metals and non-metals) or covalent bonds (between non-metals).
• Compounds can only be separated into their elements by chemical reactions, not by physical methods.
• The ratio of atoms in a compound is fixed — water is always H₂O.

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Chemical Formulae

A chemical formula shows the type and number of atoms in a substance. The small subscript number tells you how many atoms of that element are present. If there is no subscript, there is one atom.

Formula	Meaning
H₂	2 hydrogen atoms (an element, not a compound)
O₂	2 oxygen atoms
H₂O	2 hydrogen atoms and 1 oxygen atom
CO₂	1 carbon atom and 2 oxygen atoms
NaCl	1 sodium atom and 1 chlorine atom
CaCO₃	1 calcium, 1 carbon and 3 oxygen atoms

Working with Brackets in Formulae

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

• Ca(OH)₂ → 1 Ca, 2 × O = 2 O, 2 × H = 2 H → total 5 atoms
• Mg(NO₃)₂ → 1 Mg, 2 × N = 2 N, 2 × 3 O = 6 O → total 9 atoms

Exam Tip: In formulae with brackets, multiply the subscript outside by every element inside. In Mg(NO₃)₂ there are 6 oxygen atoms (3 × 2), not 3.

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Classification of Matter

graph TD
    A["All Matter"] --> B["Pure Substances"]
    A --> C["Mixtures"]
    B --> D["Elements"]
    B --> E["Compounds"]
    D --> F["One type of atom only<br/>e.g. O₂, Fe, C"]
    E --> G["Two or more different atoms<br/>chemically bonded<br/>e.g. H₂O, NaCl"]
    C --> H["Two or more substances<br/>NOT chemically bonded<br/>e.g. air, salt water"]

    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:#27ae60,color:#fff
    style F fill:#1a1a2e,color:#fff
    style G fill:#1a1a2e,color:#fff
    style H fill:#1a1a2e,color:#fff

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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 is represented by a chemical symbol on the periodic table.
• A compound contains two or more different elements chemically bonded together in fixed proportions.
• Compounds have different properties from their constituent elements.
• Chemical symbols use one or two letters (the first is always a capital, the second is always lower case).
• Chemical formulae show the number and type of atoms using subscripts.
• The periodic table arranges elements by atomic number into groups (columns) and periods (rows).

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Worked Examples — Elements, Compounds and Formulae

Worked Example 1 — Counting atoms in a formula

Question: How many atoms in total are present in one formula unit of aluminium sulfate, Al₂(SO₄)₃?

Step-by-step:

1. Aluminium: the subscript outside the Al is 2, so 2 Al atoms.
2. Inside the bracket (SO₄), there is 1 S and 4 O.
3. The subscript outside the bracket is 3, so multiply: 3 × 1 = 3 S atoms and 3 × 4 = 12 O atoms.
4. Total atoms = 2 + 3 + 12 = 17 atoms per formula unit.

Exam Tip: Always multiply the contents of the bracket by the subscript outside. A common error is to multiply only the last element inside the bracket.

Worked Example 2 — Deciding element, compound or mixture

Question: Classify each of the following as element, compound or mixture: (a) brass, (b) methane CH₄, (c) graphite, (d) muddy water.

Substance	Classification	Reasoning
Brass	Mixture	An alloy of copper and zinc; the metals are not chemically bonded
Methane (CH₄)	Compound	Two different elements (C and H) chemically bonded in a fixed ratio
Graphite	Element	Made of only carbon atoms
Muddy water	Mixture	Contains water plus insoluble soil particles, not chemically bonded

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Comparison Table — Elements vs Compounds vs Mixtures

Feature	Element	Compound	Mixture
Made of	One type of atom	Two or more elements chemically bonded	Two or more substances not bonded
Fixed ratio?	N/A (single atom type)	Yes — always fixed	No — any proportion
Separation	Cannot be broken down chemically	Only by chemical reactions (electrolysis, thermal decomposition)	By physical methods (filtration, distillation)
Melting/boiling point	Sharp	Sharp	Range
Examples	Fe, O₂, Na	H₂O, NaCl, CaCO₃	Air, sea water, brass

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Common Mistakes to Avoid

Common Mistake #1: Confusing Co (cobalt — an element) with CO (carbon monoxide — a compound). Case matters in chemical notation.

Common Mistake #2: Writing the subscript as a full-sized number, e.g. "H2O" instead of "H₂O". In handwritten work the subscript should clearly sit below the line.

Common Mistake #3: Forgetting that some elements exist as diatomic molecules — hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine are written as H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂ respectively. Writing "H" for hydrogen gas loses marks.

Common Mistake #4: Describing an alloy such as brass as a "compound" because the metals are mixed. Alloys are mixtures — the metals are not chemically bonded.

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Diagram — From Atom to Compound

graph LR
    A["Atom<br/>(single particle)"] --> B["Element<br/>(same type of atom)"]
    B --> C["Molecule of element<br/>e.g. O₂, N₂"]
    B --> D["Compound<br/>(different atoms bonded)"]
    D --> E["Molecular compound<br/>e.g. H₂O, CO₂"]
    D --> F["Ionic compound<br/>e.g. NaCl, MgO"]

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

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Exam-Style Practice with Model Answers

Question 1 (2 marks): Explain the difference between an element and a compound.

Model answer: An element is a pure substance made of only one type of atom (1 mark). A compound contains two or more different elements chemically bonded together in fixed proportions (1 mark).

Question 2 (3 marks): Air is described as a mixture. State what is meant by a mixture and name two substances found in air.

Model answer: A mixture is two or more substances that are not chemically bonded together (1 mark). Air contains nitrogen (1 mark) and oxygen (1 mark). Accept also: argon, carbon dioxide, water vapour.

Question 3 (3 marks): A student says "sodium chloride is a mixture because it contains two elements." Explain why this is wrong.

Model answer: Sodium chloride is a compound because the sodium and chlorine atoms are chemically bonded together (1 mark) in a fixed ratio of 1:1 (1 mark). The properties of sodium chloride are completely different from those of sodium metal and chlorine gas (1 mark).

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Answering at different grade levels

Examiners reward precise terminology when candidates discuss atoms, elements and compounds.

• Grade 3–4 answers typically describe atoms as "the smallest thing" and may confuse compounds with mixtures. Definitions are vague and the student rarely uses the terms atomic number, mass number or electronic configuration correctly.
• Grade 5–6 answers define an atom as "the smallest particle of an element that can take part in a chemical reaction" and correctly identify that a compound has atoms joined by chemical bonds. They can read a formula such as Ca(OH)₂ and state the number of each atom.
• Grade 7–9 answers go further: they reference the atomic number as the identifier for an element, explain that the mass number is protons + neutrons, distinguish isotopes as variants with the same atomic number but different mass numbers, and connect electronic configuration to chemical behaviour. They use "relative atomic mass" correctly and note that it is a weighted mean.

Edexcel alignment: This content is aligned with Edexcel GCSE Combined Science (1SC0) Chemistry Topic 1 Key concepts in chemistry — specifically CC1 States of matter, CC2 Methods of separating and purifying substances, CC3 Atomic structure and CC4 The periodic table. Assessed on Chemistry Paper 1.