# Calcium has atomic number 20 and mass number 40. Describe the structure of a calcium atom, including the arrangement of its subatomic particles and the distribution of its electrons in shells.

> Pearson Edexcel International GCSE Chemistry (4CH1) — 1.2 Atoms · Describe · 4 marks

## Mark scheme (4 marks)

1. The nucleus contains 20 protons (and is positively charged)
2. The nucleus contains 20 neutrons (mass number minus atomic number = 40 − 20)
3. The atom has 20 electrons which orbit/surround the nucleus (negatively charged)
4. Electrons are arranged in shells as 2, 8, 8, 2 (outermost/fourth shell contains 2 electrons)

## Key terms

- [atomic number](https://www.gradenine.co.uk/glossary/atomic-number)
- [mass number](https://www.gradenine.co.uk/glossary/mass-number)
- [electron](https://www.gradenine.co.uk/glossary/electron)

## Related

- [Revision notes for Pearson Edexcel International GCSE Chemistry (4CH1)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/calcium-has-atomic-number-20-and-4f1f8401) · Published by Druglandscape Ltd.