Atomic Structure: IGCSE Chemistry Exam Guide

Atomic structure is the foundation topic for IGCSE Chemistry. It carries direct marks on every Paper 2 and Paper 4, and it underpins bonding, the periodic table, electrochemistry, and stoichiometry. Getting this topic right gives you both the marks it carries directly and the understanding needed to answer questions across the rest of the syllabus.

The five question types

1. Subatomic particles from data

The exam gives you the atomic number (Z) and mass number (A) and asks you to state the number of protons, neutrons, and electrons.

• Protons = atomic number
• Neutrons = mass number - atomic number
• Electrons = atomic number (for a neutral atom)

For ions, adjust the electron count: a positive ion (e.g., Na+) has lost electrons (one fewer than the atomic number for a 1+ charge). A negative ion (e.g., Cl-) has gained electrons (one more than the atomic number for a 1- charge).

Examiner tip: The data may be given as notation (e.g., 23/11 Na). Read the numbers carefully — the bottom number is always the atomic number. Students who confuse top and bottom lose both marks.

2. Isotopes

Definition: atoms of the same element with the same number of protons but different numbers of neutrons.

The exam tests this in three ways:

• Define isotope (1-2 marks). Use the exact definition above.
• Identify isotopes from data (1 mark). Two atoms are isotopes if they have the same atomic number but different mass numbers.
• Calculate relative atomic mass from isotope abundances (2-3 marks, extended).

The Ar calculation uses the formula: Ar = (mass of isotope 1 x abundance 1 + mass of isotope 2 x abundance 2) / 100

Example: Chlorine has two isotopes, Cl-35 (75%) and Cl-37 (25%). Ar = (35 x 75 + 37 x 25) / 100 = (2625 + 925) / 100 = 35.5

Common error: forgetting to divide by 100 (or by the total abundance if given in non-percentage form). Always show the substitution step and the division step for full method marks.

3. Electron configuration

Draw or write the electron arrangement for an atom or ion.

Drawing: Show concentric circles (shells) with dots or crosses representing electrons. The first shell holds a maximum of 2, the second holds 8, the third holds 8 (at IGCSE level), and the fourth begins filling for elements 19 (potassium, 2,8,8,1) and 20 (calcium, 2,8,8,2).

Writing: Use the shorthand notation: sodium is 2,8,1.

The exam awards marks for:

• Correct number of electrons (must match the atomic number or the ion charge)
• Correct distribution across shells
• Filling shells in order (2, then 8, then 8, then next shell)

For ions: draw the electron configuration of the ion, not the atom. Na+ has the configuration 2,8 (it has lost one electron). Cl- has the configuration 2,8,8 (it has gained one electron). Some questions ask for both the atom and ion configurations.

Link to bonding: Electron configuration explains why atoms bond the way they do. Sodium (2,8,1) loses one electron to achieve a stable configuration (2,8), forming Na+. Chlorine (2,8,7) gains one electron (2,8,8), forming Cl-. This transfers directly to ionic bonding questions.

4. Elements, compounds, and mixtures

These definitions are tested frequently, often as MCQs or as the opening part of a structured question.

• Element: A substance containing only one type of atom. Cannot be broken down into simpler substances by chemical means.
• Compound: A substance containing two or more elements chemically combined in a fixed ratio.
• Mixture: Two or more substances not chemically combined. Can be separated by physical methods.

The exam may give you diagrams of particle arrangements and ask you to classify them. The key visual cues: an element shows only one type of particle. A compound shows particles made of two or more types of atom bonded together. A mixture shows different types of particle not bonded to each other.

5. Atoms, molecules, and ions

Distinguish between:

• An atom: the smallest particle of an element that can take part in a chemical reaction
• A molecule: two or more atoms covalently bonded together
• An ion: an atom (or group of atoms) with a charge, due to loss or gain of electrons

The exam tests whether you can identify a formula as representing an atom (Fe), a molecule (O2, H2O), or an ion (Na+, SO4 2-). Extended questions may ask you to explain how ions form from atoms, linking to electron configuration and bonding.

Mark scheme patterns

Definitions (1-2 marks each)

The mark scheme requires specific terms. “A compound is when elements are joined together” does not score. “Two or more elements chemically combined in a fixed ratio” scores both marks. Learn the definitions using the precise syllabus wording and practice writing them from memory.

Particle counts (1 mark each)

These are either right or wrong. No method marks are available. Double-check your arithmetic: protons = atomic number, neutrons = mass number minus atomic number, electrons = protons (for neutral atoms) or protons minus charge (for ions).

Electron diagrams (2-3 marks)

Marks are awarded for: correct total number of electrons (1 mark), correct distribution across shells (1 mark), and sometimes for showing the diagram clearly with shells labelled (1 mark). An electron diagram that has the right total but puts them in the wrong shells scores only the first mark.

Ar calculations (2-3 marks)

Method mark for the correct setup (multiplying each isotope mass by its abundance). Answer mark for the correct final value. If the question gives three isotopes instead of two, the method is the same — just add a third term. Show all working to secure the method mark even if your arithmetic slips.

Common errors to avoid

• Writing “same number of electrons” in the isotope definition instead of “same number of protons.” Ions of the same isotope have different electron counts, so the definition must reference protons.
• Drawing 8 electrons in the first shell (it holds only 2).
• Forgetting to adjust the electron count for ions. Ca2+ has 18 electrons, not 20.
• Confusing “molecule” and “compound.” O2 is a molecule but not a compound (it contains only one element). H2O is both a molecule and a compound. This distinction is a favourite MCQ trap.
• Giving an incomplete definition of relative atomic mass. It must reference the scale where carbon-12 = 12, not just “the average mass of an atom.”

For detailed study strategies on this topic, see the guide on how to study bonding and structure.