# 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

## Mark scheme (4 marks)

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

## Key terms

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

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/calcium-has-atomic-number-20-and-e41ad58e) · Published by Druglandscape Ltd.