Explain why two atoms of the same element can have different mass numbers, and explain why this does not change the chemical properties of those atoms.

Two atoms of the same element can have different mass numbers because they are isotopes – they contain the same number of protons but a different number of neutrons in the nucleus. The mass number is the sum of protons and neutrons, so a change in neutron number changes the mass number.

This does not alter the chemical properties because the chemical behaviour of an element is governed by its electrons. Isotopes have the same number of protons, therefore the same number of electrons, and the electron configuration is unchanged. Since chemical reactions depend on the arrangement of electrons, isotopes of the same element behave identically in chemical processes.