Isotopes — 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.

Isotopes are atoms of the same element with the same number of but a different number of
All isotopes of an element share:
- the same (so the same number of protons and electrons)
- the same chemical properties (chemistry is set by the electron count, which equals the proton count)
Isotopes differ in:
- (different neutron count)
- some physical properties like density and rate of diffusion
Isotopes are written with the mass number as a left-superscript on the element symbol:
- ¹²C, ¹³C, ¹⁴C are three isotopes of carbon
- All three have 6 protons; they have 6, 7 and 8 neutrons respectively

Why isotopes matter

Most elements in nature are a mixture of their isotopes in fixed natural proportions
This is why most relative atomic masses on the periodic table are not whole numbers — they are weighted averages over the natural isotope mix (see section 5)

Exam tip

Why isotopes of the same element have identical chemical properties

What comes up: a 1-mark question asks why two isotopes of an element react in the same way or have identical chemical properties.

Write: they have the same number of electrons and the same electron configuration, so they react in the same way (chemical behaviour is set by the electrons).

Watch out: answer in terms of electrons, not neutrons. Isotopes differ only in their number of neutrons, so a neutron-based answer does not explain why the chemistry is identical — state the same number of electrons / same electron configuration.