IGCSE Chemistry (0620)

Isotopes and relative atomic mass

What this topic tests, the mistakes that cost marks, and one exam-style question worked through step by step.

Key ideas and common mistakes

The term 'isotopes' refers to atoms with the same number of protons but different numbers of neutrons.
Common mistake: Students often confuse this with ions (which differ in electrons) or elements (which differ in protons).
The relative atomic mass equals the nucleon number only when the element has a single naturally occurring isotope.
Common mistake: Students often forget that the relative atomic mass is an average weighted by isotope abundance, not just any isotope's mass number.
To find the relative atomic mass from isotope abundances, multiply each isotope's mass by its percentage abundance, sum these, and divide by 100.
Common mistake: A common mistake is to average the masses without weighting by abundance.
Isotopes of an element have identical chemical properties because they have the same number and arrangement of electrons.
Common mistake: Students who focus on the different number of neutrons may wrongly think this affects chemical reactivity.
To find the percentage abundance of two isotopes given their mass numbers and the relative atomic mass, set up and solve a weighted average equation.
Common mistake: The common mistake is to average the mass numbers directly or misassign the weights.
Define isotopes as same protons but different neutrons, then link chemical properties to electrons: equal protons mean equal electrons and electron arrangement, and reactions depend on those, not neutrons.

State the term used for atoms of the same element that contain different numbers of neutrons.

Try it first, then show the working

Answer: Isotopes

18 exam-style questions on Isotopes and relative atomic mass, each marked to the scheme.