A neutral atom of oxygen has a proton number of 8 and a nucleon number of 16. State the number of protons, neutrons, and electrons in this atom. Then explain how a negative ion of oxygen is formed from this neutral atom.

Cambridge International IGCSE Physics (0625) — 5.1 The nuclear model of the atom · State and Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Protons: 8
Neutrons: 8 (16−8)
Electrons: 8
A negative ion is formed when the atom gains one or more electrons, giving it an overall negative charge.

Use the command word ‘state’ to give the exact numbers first, then ‘explain’ the ionisation process.
Show the calculation for neutrons (16−8) to demonstrate understanding of nucleon number.
Mention that the extra electrons give the ion a negative charge.
Keep the answer concise and use correct terminology (proton, neutron, electron, ion).

Confusing neutrons with protons or giving the wrong number of neutrons.
Failing to explain that the ion is negative because it has more electrons than protons.
Using vague language such as ‘more electrons’ without stating the charge change.

8 protons
8 neutrons (nucleon number minus proton number = 16 − 8)
8 electrons (equal to number of protons in a neutral atom)
A negative ion is formed when the atom gains electrons, giving it an overall negative charge