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.

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.

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

The first letter is always a capital letter.
If there is a second letter, it is always lower case.
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.

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.

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.

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.

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

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).