Calcium has atomic number 20 and mass number 40. Describe the arrangement of subatomic particles in one atom of calcium, including the number of each type of subatomic particle and where they are located.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.3 Atomic structure · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

The nucleus contains 20 protons and 20 neutrons. The 20 electrons are arranged in shells around the nucleus in the order 2, 8, 8, 2.

Use the exact wording ‘nucleus contains 20 protons’ and ‘20 neutrons’ to match the mark scheme.
State the electron configuration as 2, 8, 8, 2 to show the shell arrangement.
Mention that electrons are located in shells surrounding the nucleus.
Keep the answer concise – 4 marks only.

Writing ‘20 electrons in 2, 8, 8, 2 shells’ without specifying they are in the nucleus or shells.
Confusing the number of neutrons with the mass number.
Using the wrong electron configuration such as 2, 8, 8, 1 or 2, 8, 10.

The nucleus contains 20 protons (positively charged subatomic particles)
The nucleus contains 20 neutrons (mass number minus atomic number = 40 − 20)
There are 20 electrons, located in shells / energy levels surrounding / orbiting the nucleus
Electrons are arranged in shells as 2, 8, 8, 2 (correct electron configuration)