GCSE Chemistry Glossary (AQA & Edexcel): Step-by-Step Definitions & Examples

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3 E
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5 M
6 N
7 P
GCSE Chemistry Glossary

A

Atom (AQA & Edexcel GCSE Chemistry Glossary Step-by-Step Definition)

What is an Atom?

The basic building block of all matter—everything is made of atoms. An atom is the smallest part of an element that still has the properties of that element.

The Parts of an atom:

Protons – positive charge
Neutrons – no charge
Electrons – negative charge

Where are these parts found?

Protons and neutrons: In the nucleus (centre of the atom)
Electrons: In shells/energy levels around the nucleus

Why do atoms matter in chemistry?

Atoms combine to form molecules and compounds. Understanding atoms is the foundation for all other topics in chemistry.

Exam tip (AQA & Edexcel):

You may need to label or draw an atom, or explain its structure. Focus on:

The number of protons (atomic number)
The arrangement of electrons (shells/energy levels)

See also:

Atomic Number (AQA & Edexcel GCSE Chemistry Step-by-Step Definition)

What is the Atomic Number?

The atomic number is the number of protons in the nucleus of an atom. It is unique to each element and determines the element’s identity and position on the periodic table.

What you need too know about the atomic number:

The atomic number is always a whole number and is shown above the element symbol on the periodic table.

All atoms of the same element have the same atomic number (same number of protons).
The atomic number decides which element an atom is (e.g., atomic number 6 = carbon).
In a neutral atom, the atomic number also tells you the number of electrons.

Why does atomic number matter in chemistry?

The atomic number helps you identify elements, write electronic configurations, and understand chemical properties. It’s the foundation for understanding atomic structure and the periodic table.

Exam tip (AQA & Edexcel):

You may be asked to find the atomic number from the periodic table or use it to work out the number of protons/electrons in an atom.
Remember: Atomic number = number of protons = number of electrons (in a neutral atom).

See also:

Mass Number
Periodic Table

C

Compound (AQA & Edexcel GCSE Chemistry Step-by-Step Definition)

What is a Compound?

A compound is a substance made from two or more different elements that are chemically joined together.

How compounds form

Atoms of different elements react and form chemical bonds
The properties of a compound are different from the elements it’s made from.
Compounds have chemical formulas (e.g., H₂O for water, CO₂ for carbon dioxide)

Why do compounds matter in chemistry?

Understanding compounds helps you explain the properties and behaviours of most substances around you

Exam tip (AQA & Edexcel):

Be able to write and interpret chemical formulas, and explain how compounds differ from mixtures.

See also:

Chemical Bond

E

Electron (AQA & Edexcel GCSE Chemistry Step-by-Step Definition)

What is an Electron?

An electron is a tiny particle that moves around the nucleus of an atom. It has a negative electric charge

How to understand electrons

Electrons are found in shells (energy levels) around the nucleus.
Each electron has a single negative charge (–1).
The number of electrons in a neutral atom equals the number of protons.
Electrons can be gained or lost to form ions.

Why do electrons matter in chemistry?

Electrons are involved in chemical bonding and reactions. Their arrangement explains how atoms interact and combine.

Exam tip (AQA & Edexcel):

You may need to draw electron shells or explain electron arrangement.
Remember: Number of electrons = number of protons (in a neutral atom).

See also:

Electron Shell

Element (AQA & Edexcel GCSE Chemistry Step-by-Step Definition)

What is an Element?

An element is a pure substance made of only one type of atom.

How to understand elements

Each element has its own symbol (e.g., H for hydrogen, O for oxygen)
Elements are listed on the periodic table
Elements cannot be broken down into simpler substances by chemical reactions

Why do elements matter in chemistry?

Elements are the building blocks for all compounds and materials you encounter.

Exam tip (AQA & Edexcel):

Know how to use the periodic table to find element symbols, atomic numbers, and properties.

See also:

Periodic Table

I

Ion (AQA & Edexcel GCSE Chemistry Step-by-Step Definition)

What is an Ion?

An ion is an atom or molecule that has gained or lost one or more electrons, giving it an electrical charge.

How do ions form?

Start with a neutral atom (equal numbers of protons and electrons)
Lose electrons: Becomes a positive ion (cation)
Gain electrons: Becomes a negative ion (anion)

Why do ions matter in chemistry?

Ions are crucial for understanding chemical reactions, electricity in solutions, and many real-life processes (e.g., table salt, batteries).

Exam tip (AQA & Edexcel):

Be able to explain how ions form and recognise the difference between cations and anions.

See also:

Cation
Anion

Isotope (AQA & Edexcel GCSE Chemistry Step-by-Step Definition)

What is an Isotope?

An isotope is a form of an element’s atom that has the same number of protons but a different number of neutrons in its nucleus. Isotopes of the same element have the same chemical properties but different mass numbers.

How to understand Isotopes

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons.)
Isotopes have the same atomic number but different mass numbers.
Most elements have more than one isotope (e.g., carbon-12 and carbon-14)
Isotopes behave the same in chemical reactions but may differ in physical properties (like stability or radioactivity).

Why do molecules matter in chemistry?

Isotopes help explain atomic mass, radioactive decay, and are used in medicine, archaeology (carbon dating), and nuclear power.

Exam tip (AQA & Edexcel):

You may be asked to calculate the number of neutrons in an isotope: Mass number – Atomic number = Number of neutrons.
You could be asked to explain why isotopes have similar chemical properties but different masses.

See also:

Mass Number
Radioactivity

M

Molecule (AQA & Edexcel GCSE Chemistry Step-by-Step Definition)

What is an Molecule?

A molecule is two or more atoms joined (bonded) together.

How to understand elements

Molecules can be made from the same element (e.g., O₂) or different elements (e.g., H₂O)
The way atoms are bonded determines the molecule’s properties
Molecules can be small or large (from simple gases to complex proteins)

Why do molecules matter in chemistry?

Molecules explain how atoms combine and interact in chemical reactions and in living things

Exam tip (AQA & Edexcel):

Know how to draw simple molecules, recognize molecular formulas, and explain differences between molecules and compounds.

See also:

Periodic Table

N

Neutron (AQA & Edexcel GCSE Chemistry Step-by-Step Definition)

What is a Neutron?

A neutron is a tiny particle found inside the nucleus of an atom. It has no electric charge (it is neutral).

Key Facts about protons

Neutrons are found in the nucleus, along with protons.
Each neutron has a single no charge (0)
The number of neutrons can vary in atoms of the same element—this creates isotopes.
Neutrons help hold the nucleus of an atom together

Why do neutrons matter in chemistry?

Neutrons affect the mass of an atom and create isotopes, which are important for understanding atomic structure and radioactivity.

Exam tip (AQA & Edexcel):

You may need to calculate the number of neutrons:
Number of neutrons = Mass number – Atomic number.

See also:

P

Proton (AQA & Edexcel GCSE Chemistry Step-by-Step Definition)

What is a Proton?

A proton is a tiny particle found inside the nucleus (centre) of an atom. It has a positive electric charge.

How to understand protons

Protons are found in the nucleus, together with neutrons.
Each proton has a single positive charge (+1)
The number of protons in an atom is called the atomic number—it decides which element the atom is.
All atoms of the same element have the same number of protons

Why do protons matter in chemistry?

Protons decide the identity of an element (e.g., all hydrogen atoms have 1 proton, all carbon atoms have 6 protons).

Exam tip (AQA & Edexcel):

You may be asked to state the number of protons in an atom or use the atomic number from the periodic table.
Remember: Atomic number = number of protons.

See also:

Atomic Number