2.2

Atomic structure

Cambridge IGCSE Chemistry (0620)  · Unit 2: Atoms, elements and compounds  · 12 flashcards

Atomic structure is topic 2.2 in the Cambridge IGCSE Chemistry (0620) syllabus , positioned in Unit 2 — Atoms, elements and compounds , alongside Elements, compounds and mixtures, Bonding and Structure and properties.  In one line: An atom consists of a central nucleus containing protons and neutrons, surrounded by electrons in shells (energy levels).

This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical). Past papers from 2022 to 2025 show this topic across undefined questions worth 112 marks (around 1.7% of all Chemistry marks in those years).

The deck below contains 12 flashcards — 4 definitions, 5 key concepts and 3 identification cards — covering the precise wording mark schemes reward.  Use the 4 definition cards to lock down command-word answers (define, state), then move on to the concept and application cards to handle explain, describe and compare questions.

Key definition

Describe the structure of an atom

An atom consists of a central nucleus containing protons and neutrons, surrounded by electrons in shells (energy levels).

What the Cambridge 0620 syllabus says

2026-2028 syllabus, in our words

What the syllabus expects you to be able to do in this topic, in plain English. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks. For the exact wording, see the official syllabus (PDF).

  1. Describe Describe the atom's structure: a nucleus at the centre made of protons and neutrons, with electrons arranged in shells around it.
  2. State State the relative charge and relative mass of each of the proton, neutron and electron.
  3. Define Define atomic number (also called proton number): how many protons an atom has in its nucleus.
  4. Define Define nucleon number (also known as mass number): the total count of protons plus neutrons in an atom's nucleus.
  5. Determine Determine, for elements with proton numbers 1 to 20 and their ions, the electronic configuration (written like 2,8,3).
  6. State State that: (a) the noble gases in Group VIII have a complete outer shell of electrons; (b) for Groups I to VII, the group number equals the number of electrons in the outer shell; (c) the period number equals how many electron shells are occupied.
Definition Flip

Describe the structure of an atom.

Answer Flip

An atom consists of a central nucleus containing protons and neutrons, surrounded by electrons in shells (energy levels).

Key Concept Flip

What are the relative charges of a proton, neutron, and electron?

Answer Flip

A proton has a relative charge of +1, a neutron has a relative charge of 0, and an electron has a relative charge of -1.

Example: a sodium ion (Na+) has lost one electron, resulting in a +1 charge.
Key Concept Flip

2.2 Atomic structure: What are the relative masses of a proton, neutron, and electron?

Answer Flip

Protons and neutrons have a relative mass of 1. Electrons have a negligible mass compared to protons and neutrons; its relative mass is approximately 1/1836. The actual mass of an electron is approximately 9.11 × 10⁻³¹ kg, demonstrating it is not truly massless but significantly smaller than the mass of a proton or neutron.

Definition Flip

Define proton number (atomic number).

Answer Flip

The proton number is the number of protons in the nucleus of an atom. It uniquely identifies an element.

Example: Hydrogen has 1 proton.
Definition Flip

Define mass number (nucleon number).

Answer Flip

The mass number is the total number of protons and neutrons in the nucleus of an atom.

Example: Oxygen-16 has a mass number of 16.
Key Concept Flip

What is the electronic configuration of Potassium (proton number 19)?

Answer Flip

The electronic configuration of Potassium (K) is 2,8,8,1.

Key Concept Flip

Topic: 2.2 Atomic structure Question: What is the electronic configuration of the Oxide ion (O²⁻, Oxygen has proton number 8)?

Answer Flip

An oxygen atom has electronic configuration 2,6. An O²⁻ ion has gained 2 electrons to achieve a full outer shell, similar to the noble gas Neon. Therefore, the electronic configuration of the oxide ion is 2,8.

Key Concept Flip

What is the electronic configuration of Sodium (Na, proton number 11)

Answer Flip

The electronic configuration of Sodium (Na) is 2,8,1.

Key Concept Flip

What feature do noble gases have in common?

Answer Flip

Noble gases (Group 0/VIII) have a full outer electron shell, making them stable and unreactive. This electronic arrangement means they do not readily form chemical bonds.

Example: Helium (He) has 2 electrons (a full outer shell for n=1), while Neon (Ne) has 8 electrons in its outer shell.
Key Concept Flip

How is the number of outer shell electrons related to the group number (Groups I to VII)?

Answer Flip

The number of outer shell electrons is equal to the group number.

Example: Sodium (Group I) has 1 outer shell electron.
Key Concept Flip

How is the number of occupied electron shells related to the period number?

Answer Flip

The number of occupied electron shells is equal to the period number.

Example: Sodium (Period 3) has 3 occupied electron shells (2,8,1).
Definition Flip

What are isotopes?

Answer Flip

Atoms of the same element with the same proton number but different mass numbers.

Example: Carbon-12 and Carbon-14 both have 6 protons, but C-12 has 6 neutrons while C-14 has 8.

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2.1 Elements, compounds and mixtures 2.3 Bonding

Key Questions: Atomic structure

Describe the structure of an atom.

An atom consists of a central nucleus containing protons and neutrons, surrounded by electrons in shells (energy levels).

Define proton number (atomic number).

The proton number is the number of protons in the nucleus of an atom. It uniquely identifies an element.

Example: Hydrogen has 1 proton.
Define mass number (nucleon number).

The mass number is the total number of protons and neutrons in the nucleus of an atom.

Example: Oxygen-16 has a mass number of 16.
What are isotopes?

Atoms of the same element with the same proton number but different mass numbers.

Example: Carbon-12 and Carbon-14 both have 6 protons, but C-12 has 6 neutrons while C-14 has 8.

Tips to avoid common mistakes in Atomic structure

More topics in Unit 2 — Atoms, elements and compounds

Atomic structure sits alongside these Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.

2.1 Elements, compounds and mixtures

11 flashcards
2.3 Bonding

1 flashcards
2.4 Structure and properties

9 flashcards

Cambridge syllabus keywords to use in your answers

These are the official Cambridge 0620 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.

atom proton neutron electron nucleus electron shell energy level proton number atomic number nucleon number mass number isotope relative atomic mass electronic configuration electron arrangement

Key terms covered in this Atomic structure deck

Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.

Describe the structure of an atom
Proton number (atomic number)
Mass number (nucleon number)
Isotopes

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How to study this Atomic structure deck

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