Atoms, Elements, Compounds and Mixtures

This lesson covers the fundamental building blocks of chemistry — atoms, elements, compounds and mixtures — as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 1: Key Concepts in Chemistry. You need to understand what each of these terms means, how they relate to one another, and how to use chemical symbols and formulae correctly.

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 (nm) in diameter, where 1 nm = 1 × 10⁻⁹ m.

Key facts about atoms:

Atoms are the building blocks of all matter — every substance you encounter is made of atoms.
Each element is made of its own unique type of atom, defined by the number of protons in its nucleus.
Atoms cannot be broken down into simpler substances by ordinary chemical reactions.
There are approximately 118 known elements, each with a different type of atom.

Exam Tip: When asked to define an atom in the exam, state that 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 (protons, neutrons and electrons).

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 unique chemical symbol on the periodic table.

Common Elements and Their Symbols

Element	Symbol	Type
Hydrogen	H	Non-metal
Helium	He	Non-metal (noble gas)
Carbon	C	Non-metal
Nitrogen	N	Non-metal
Oxygen	O	Non-metal
Sodium	Na	Metal
Magnesium	Mg	Metal
Chlorine	Cl	Non-metal
Iron	Fe	Metal
Copper	Cu	Metal
Gold	Au	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), Cu from cuprum (copper).

Exam Tip: Be very careful with capital and lower case letters. "CO" means carbon monoxide (a compound of carbon and oxygen), but "Co" means cobalt (a single element). Getting this wrong in an exam will lose you marks.

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 (between metals and non-metals) or covalent bonds (between non-metals).

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
Sulfuric acid	H₂SO₄	Hydrogen, Sulfur, Oxygen

Key Points About 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 (table salt) is a harmless white solid.
Compounds can only be separated into their elements by chemical reactions, not by physical methods like filtration or distillation.
The ratio of atoms in a compound is fixed — water is always H₂O, never H₃O or HO₂.

Exam Tip: A very common exam question asks you to explain the difference between a compound and a mixture. Remember: in a compound the elements are chemically bonded in fixed proportions, whereas in a mixture the substances are not chemically bonded and can be in any proportion.

Mixtures

A mixture contains two or more substances that are not chemically bonded together. The components of a mixture retain their own individual properties and can be separated by physical methods.

Comparing Compounds and Mixtures

Feature	Compound	Mixture
Chemical bonds	Elements are chemically bonded	Substances are NOT chemically bonded
Fixed ratio	Always a fixed ratio of atoms	Can be in any proportion
Properties	Different from the constituent elements	Each substance retains its own properties
Separation	Only by chemical reactions	By physical methods (filtration, distillation, chromatography, evaporation)

Methods of Separating Mixtures

Filtration — separates an insoluble solid from a liquid (e.g. sand from water).
Evaporation — removes the solvent to leave behind a dissolved solid (e.g. obtaining salt from salt water).
Distillation — separates liquids with different boiling points, or a solvent from a solution.
Chromatography — separates dissolved substances that move at different rates through a stationary phase.

Chemical Formulae

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

Formula	Atoms Present
H₂	2 hydrogen atoms
O₂	2 oxygen atoms
H₂O	2 hydrogen atoms and 1 oxygen atom
CO₂	1 carbon atom and 2 oxygen atoms
H₂SO₄	2 hydrogen, 1 sulfur and 4 oxygen atoms
Ca(OH)₂	1 calcium, 2 oxygen and 2 hydrogen atoms
Mg(NO₃)₂	1 magnesium, 2 nitrogen and 6 oxygen atoms

Working with Brackets

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

Ca(OH)₂ → 1 Ca, 2 × O = 2 O, 2 × H = 2 H
Mg(NO₃)₂ → 1 Mg, 2 × N = 2 N, 2 × 3 O = 6 O

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

Classification of Matter — Overview

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.
A compound contains two or more different elements chemically bonded together in fixed proportions.
A mixture contains two or more substances that are not chemically bonded.
Chemical symbols use one or two letters (first always capital, second always lower case).
Chemical formulae show the number and type of atoms in a substance using subscripts.
Compounds have different properties from their constituent elements.
Mixtures can be separated by physical methods; compounds require chemical reactions to separate.