OxfordAQA IGCSE Chemistry: Atomic Structure and the Periodic Table — Revision Notes

Condensed for the final weeks. For the full explanation, use the Atomic Structure and the Periodic Table study guide.

The atom (3.1.2)

Particle	Relative charge	Relative mass
Proton	+1	1
Neutron	0	1
Electron	−1	Very small

Atomic number = number of protons. Mass number = protons + neutrons.

Protons   = atomic number
Electrons = protons (neutral atom)
Neutrons  = mass number - atomic number

Isotopes — atoms of the same element (same atomic number/protons) with different neutron numbers (different mass number).

Worked example: finding sub-atomic particles

Chlorine, ³⁵₁₇Cl (mass number 35, atomic number 17):

Protons:   17 (= atomic number)
Electrons: 17 (= protons, neutral atom)
Neutrons:  35 - 17 = 18

An isotope ³⁷₁₇Cl has the same 17 protons/electrons but 37 − 17 = 20 neutrons — two more, making it an isotope of the same element (chlorine), not a different element.

Electron structure and the periodic table (3.1.3)

Shells fill 2, 8, 8 (with exceptions from element 19 onwards). Group number (1–7) = number of outer-shell electrons. Sodium (2,8,1) is Group 1 with one outer electron. Group 0 (noble gases) are unreactive — stable outer shells (8 outer electrons, except helium with 2). Other groups react to achieve a noble-gas-like stable arrangement — the logic behind ionic and covalent bonding.

States of matter (3.1.1)

Solid → liquid → gas as particles gain energy and move further apart, overcoming the forces holding them together. Diffusion experiments (e.g. bromine vapour spreading through air; ammonia and HCl gases meeting to form a white ring of ammonium chloride) are used as evidence of constant, random particle motion.

Worked example: electron structure and group number

Sodium has atomic number 11. What is its electron structure, and which group is it in?

Step 1: total electrons = atomic number = 11
Step 2: fill shells in order 2, 8, then the remainder
        Shell 1: 2 electrons (full)
        Shell 2: 8 electrons (full)
        Shell 3: 11 - 2 - 8 = 1 electron
Step 3: electron structure = 2,8,1
Step 4: outer-shell electrons (1) = group number -> sodium is Group 1

This shell-filling method (2, then 8, then the remainder) works directly for all of the first twenty elements named in this sub-topic, and the final step – reading the group number straight off the outer-shell electron count – is the specification’s own worked example, so it is worth practising until automatic rather than looking up each element’s structure individually.

Why noble gases explain reactivity elsewhere

The specification frames Group 0’s stability as the reason other groups react the way they do, not as an isolated fact about one group. Group 1 elements (one outer electron) react by losing that single electron to achieve a noble-gas-like full outer shell; Group 7 elements (seven outer electrons) react by gaining one electron for the same reason. Understanding this underlying “drive toward a stable outer shell” is what makes ionic bonding (covered in the next topic) make sense as a consequence of atomic structure, rather than a separate, unconnected rule to memorise.

Key terms

Atomic number — the number of protons in an atom, which also equals its number of electrons in a neutral atom. Mass number — protons plus neutrons. Isotope — an atom of the same element with a different number of neutrons (different mass number, same atomic number). Relative atomic mass (Ar) — an average mass compared to ¹²C. Noble gas — an unreactive Group 0 element with a stable, full outer electron shell.

Common mistakes

• Confusing atomic number (protons only) with mass number (protons + neutrons).
• Describing isotopes as having “different atomic numbers” instead of different neutron numbers.
• Forgetting a neutral atom has equal protons and electrons.
• Miscounting electron shells by not filling the lowest energy level first.

Quick self-test

1. Calculate the number of protons, electrons and neutrons in ²⁷₁₃Al.
2. Define an isotope precisely.
3. Why are noble gases unreactive?
4. What does a diffusion experiment provide evidence for?
5. What is an element’s group number equal to, for Groups 1–7?

Answers: 1. 13 protons, 13 electrons, 27 − 13 = 14 neutrons. 2. An atom of the same element (same atomic number/proton count) with a different number of neutrons, and therefore a different mass number. 3. Because they already have a stable, full outer electron shell (8 outer electrons, except helium with 2), so they have no tendency to gain, lose or share electrons. 4. That particles are in constant, random motion. 5. The number of electrons in its outer shell.

Why this topic comes first

The specification itself introduces Topic 1 by noting atoms are the building blocks of all materials, and this is the one topic in 9202 with no numerical prerequisite from elsewhere in the course – making it the natural starting point for a first pass through the specification. Everything from bonding (Topic 2) to acids and salts (Topic 5) to energy changes (Topic 9) assumes fluent recall of what an atom is made of and how the periodic table is organised, so time spent making the proton/neutron/electron calculation and the shell-filling method automatic here saves time across every later topic.

Related resources

• Atomic Structure and the Periodic Table study guide
• Atomic Structure and the Periodic Table practice questions

Official syllabus

OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2 — oxfordaqa.com/9202.