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.
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
Official 2026-2028 specThese are the exact learning objectives Cambridge sets for this topic. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks.
- Describe Describe the structure of the atom as a central nucleus containing neutrons and protons surrounded by electrons in shells
- State State the relative charges and relative masses of a proton, a neutron and an electron
- Define Define proton number/atomic number as the number of protons in the nucleus of an atom
- Define Define mass number/nucleon number as the total number of protons and neutrons in the nucleus of an atom
- Determine Determine the electronic configuration of elements and their ions with proton number 1 to 20, e.g. 2,8,3
- State State that: (a) Group VIII noble gases have a full outer electron shell (b) the number of outer shell electrons is equal to the group number in Groups I to VII (c) the number of occupied electron shells is equal to the period number
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 are the relative charges of a proton, neutron, and electron?
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.
2.2 Atomic structure: What are the relative masses of a proton, neutron, and electron?
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.
Define proton number (atomic number).
The proton number is the number of protons in the nucleus of an atom. It uniquely identifies an element.
Define mass number (nucleon number).
The mass number is the total number of protons and neutrons in the nucleus of an atom.
What is the electronic configuration of Potassium (proton number 19)?
The electronic configuration of Potassium (K) is 2,8,8,1.
Topic: 2.2 Atomic structure Question: What is the electronic configuration of the Oxide ion (O²⁻, Oxygen has proton number 8)?
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.
What is the electronic configuration of Sodium (Na, proton number 11)
The electronic configuration of Sodium (Na) is 2,8,1.
What feature do noble gases have in common?
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.
How is the number of outer shell electrons related to the group number (Groups I to VII)?
The number of outer shell electrons is equal to the group number.
How is the number of occupied electron shells related to the period number?
The number of occupied electron shells is equal to the period number.
What are isotopes?
Atoms of the same element with the same proton number but different mass numbers.
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.
Define mass number (nucleon number).
The mass number is the total number of protons and neutrons in the nucleus of an atom.
What are isotopes?
Atoms of the same element with the same proton number but different mass numbers.
Tips to avoid common mistakes in Atomic structure
- ● Clarify the distinction: electrons are the particles, shells are the energy levels they inhabit.
- ● Know that an element's identity depends on its proton number, and that the nucleon number adds protons and neutrons.
- ● Review how the charge of an ion (positive or negative) affects its number of electrons relative to the neutral atom.
- ● Practice drawing all possible structural isomers for a given molecular formula (e.g., C4H10, C5H12, C4H8).
- ● Link physical properties directly to structure: low boiling points mean weak intermolecular forces and simple molecular structures.
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.
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.
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.
Related Chemistry guides
Long-read articles that go beyond the deck — cover the whole subject's common mistakes, high-yield content and revision pacing.
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