Chemistry 0620 · syllabus 2.1, 2.2, 2.3
Cambridge IGCSE Chemistry 0620 — Atomic structure topical past papers
Elements, compounds and mixtures; protons, neutrons and electrons; proton and mass numbers; electronic configuration; isotopes and relative atomic mass.
- Questions
- 543
- Years
- 2009–2026
- With mark scheme
- 99%
- Questions 2022–2026
- 227
Anyone can browse these pages. Opening the questions and their mark schemes takes a free Momentum account.
What Atomic structure covers
An element is made of one kind of atom; a compound contains two or more elements chemically combined in a fixed ratio; a mixture contains substances that are not chemically combined and can be separated by physical methods. Deciding which is which, from a description or a diagram of particles, is the first skill the topic tests, and it rests on knowing what is inside an atom.
Every atom has a tiny central nucleus of protons and neutrons, surrounded by electrons in shells. A proton has a relative charge of +1 and a relative mass of 1, a neutron no charge and a relative mass of 1, and an electron a charge of −1 and a relative mass of about 1/1840. The proton number (atomic number) identifies the element, and the mass number (nucleon number) counts protons and neutrons together. For the first twenty elements you write electronic configurations such as 2,8,3 for aluminium: the number of occupied shells is the period, the outer-shell electrons give the group for Groups I to VII, and the noble gases have full outer shells. Ions follow from the same counting, so Al³⁺ is 2,8.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, written with the mass number above the proton number, as in ³⁵₁₇Cl. On the Extended route you explain why the isotopes of an element react in the same way — they have the same number of electrons and so the same electronic configuration — and calculate a relative atomic mass from isotopic masses and abundances. The counting skills of this topic are used again in bonding, the Periodic Table and stoichiometry.
In the syllabus
The headings this topic has in the Cambridge International 2026–2028 syllabus, as the board prints them.
- 2.1 Elements, compounds and mixtures
- 2.2 Atomic structure and the Periodic Table
- 2.3 Isotopes
What you should be able to do
- Tell elements, compounds and mixtures apart from descriptions and particle diagrams
- State the relative charges and relative masses of protons, neutrons and electrons
- Work out the numbers of protons, neutrons and electrons in an atom or ion from its symbol
- Write the electronic configurations of the first twenty elements and their ions, and link them to group and period
- Define isotopes and interpret symbols for atoms and ions such as ¹²₆C and ³⁵₁₇Cl⁻
- Explain why isotopes have the same chemical properties, and calculate a relative atomic mass from isotopic abundances (Extended)
Key ideas
- Proton: relative charge +1, relative mass 1; neutron: 0 and 1; electron: −1 and about 1/1840
- Number of neutrons = mass number − proton number; a positive ion has lost electrons and a negative ion has gained them
- For the first twenty elements the shells hold 2, 8 and 8 electrons before the fourth begins, so potassium is 2,8,8,1
- Outer-shell electrons = group number for Groups I to VII; number of occupied shells = period number
- Relative atomic mass is a weighted average: Ar = Σ(isotopic mass × percentage abundance) ÷ 100
How Atomic structure has been examined
From 2022 to 2026, Atomic structure was on Paper 3 · Theory (Core) in 14 of 14 exam series, worth about 20% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 1 · Multiple Choice (Core) | 79 | 6% | 14 of 14 |
| Paper 2 · Multiple Choice (Extended) | 64 | 5% | 14 of 14 |
| Paper 3 · Theory (Core) | 48 | 20% | 14 of 14 |
| Paper 4 · Theory (Extended) | 32 | 15% | 12 of 14 |
| Paper 6 · Alternative to Practical | 4 | 2% | 2 of 14 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1.
Filed in Momentum under: Atoms and Elements.
A worked example
Boron, proton number 5, has two isotopes, boron-10 and boron-11, with abundances of 20.0% and 80.0%. State the numbers of protons, neutrons and electrons in an atom of each isotope, explain why the two isotopes react in the same way, and calculate the relative atomic mass of boron.
- Both isotopes have 5 protons and, as neutral atoms, 5 electrons
- Neutrons = mass number − proton number: boron-10 has 10 − 5 = 5 and boron-11 has 11 − 5 = 6
- Both atoms have the electronic configuration 2,3; reactions depend on electrons, so the isotopes react in the same way
- Ar = (10 × 20.0 + 11 × 80.0) ÷ 100 = (200 + 880) ÷ 100 = 10.8
- Check: 10.8 lies between 10 and 11 and nearer 11, the more abundant isotope
Answer: Each has 5 protons and 5 electrons; boron-10 has 5 neutrons and boron-11 has 6; both are 2,3, so they react alike; Ar = 10.8
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Forgetting to adjust the electrons in an ion — Mg²⁺ has 12 protons but only 10 electrons, 2,8, because it has lost two
- Writing potassium as 2,8,9 — the fourth shell starts after eight electrons in the third, so it is 2,8,8,1, which also matches its place in Group I
- Treating relative atomic mass as the mass number of one atom — Ar averages all the isotopes by abundance, which is why chlorine's is 35.5
- Saying isotopes have different chemical properties — they have the same number of electrons in the same arrangement, so they react in the same way
- Giving the electron a relative mass of zero — it is tiny, about 1/1840 (sometimes rounded to 1/2000), but not zero
- Calling air a compound because it contains several elements — its gases are mixed, not chemically combined, and they can be separated physically
Exam technique
- Set out protons, neutrons and electrons as a small table before you answer; it catches slips with ions and isotopes
- Check every electronic configuration by adding the electrons: they should equal the proton number, minus the charge for a positive ion or plus it for a negative ion
- In a relative atomic mass calculation, write the whole expression with every mass and abundance, give the answer to one decimal place unless told otherwise, and check it lies nearer the more abundant isotope
- When asked why isotopes react the same way, give both halves of the reason: the same number of electrons, and therefore the same electronic configuration
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
Past-paper questions and mark schemes are the copyright of Cambridge International. Momentum shows them to signed-in learners for revision and does not publish them on these pages.