Atomic structure

What you will learn

• Describe the key chemistry ideas behind atomic structure.
• Use precise GCSE command-word language in explanations.
• Apply the idea to unfamiliar substances, data or practical contexts.
• Check answers using units, state symbols, observations or balanced equations where relevant.

Core knowledge

• Atoms contain protons and neutrons in the nucleus, with electrons arranged in shells around the nucleus.
• The atomic number is the number of protons. In a neutral atom, the number of electrons equals the number of protons.
• The mass number is the number of protons plus neutrons, so neutrons = mass number - atomic number.
• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
• Ions form when atoms gain or lose electrons. Losing electrons forms positive ions; gaining electrons forms negative ions.

Atomic structure and periodic table infographic

Atomic Structure practice set

Use the worked examples and practice questions on this page as a complete study task: learn the proton and neutron meanings, summarise the infographic in your own words, then answer the questions using the data, equations and observations given here. Check every answer for relative mass, atomic number, mass number and isotope abundance.

Clear explanation

Atomic structure is the language behind the periodic table, bonding and ions. GCSE questions often give an atomic number and mass number, then ask you to work out particle numbers.

The atomic number identifies the element because every element has its own proton number. If the proton number changes, the element changes.

Isotopes do not have different numbers of protons. They have different numbers of neutrons, which changes the mass number but not the chemical identity.

Worked examples

Quick checks

Choose an answer, then check your thinking.

Practice questions

Practice ladder

Answers and marking guidance

The exact practice answers are hidden under each question so you can try first. Marks come from using the correct chemistry model, choosing the right equation where needed, keeping units with values, and explaining changes with precise words such as particles, ions, bonds, energy, rate, evidence and uncertainty.

Common mistakes

• Confusing mass number with relative atomic mass.
• Saying isotopes have different numbers of protons.
• Forgetting neutral atoms have equal protons and electrons.
• Writing that ions form by gaining or losing protons.

Extension challenge

Create a six-line revision card for atomic structure: two definitions, one infographic detail or equation line, one equation or data check, one common mistake, and one complete explanation sentence using proton, neutron, electron.

Exam-board guidance

Short board notes only. Learn the core chemistry above first.

Next lesson

Next, continue with Periodic table patterns.