Atomic Number and Mass Number — Atomic Structure | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 9% of your exam marks.

stable

Medium

Stable9%

Subatomic particles and electronic configuration appear in every exam series.

(Z): the number of in the nucleus of an atom
- Each element has its own unique atomic number — no two elements share one
- In a neutral atom, the number of electrons also equals Z
(A): the total number of protons + in the nucleus

Counting subatomic particles

Number of protons = Z
Number of = Z (for a neutral atom)
Number of neutrons = A − Z
Example. An atom of beryllium has atomic number 4 and mass number 9, so it has:
- 4 protons
- 4 electrons
- 9 − 4 = 5 neutrons

Reading the periodic table

Both Z and A are given alongside every element on the periodic table
The larger of the two numbers is the mass number
The smaller is the atomic number

Exam tip

Atomic number vs mass number

Defining atomic number or mass number is a recurring 1-marker, so you need to know: atomic (proton) number = the number of protons in the nucleus; mass number = protons + neutrons. Define them in the nucleus only — "number of protons and electrons" is not credited.