# Magnesium (Mg) is in Period 3 and Group 2 of the periodic table. Silicon (Si) is also in Period 3 but in Group 4. Explain how the positions of magnesium and silicon in the periodic table reflect the structure of their atoms, and describe one trend you would expect to observe as you move across Period 3 from left to right.

> OCR A-Level Chemistry A (H432) — 3.1 The periodic table and periodicity · Explain · 5 marks

> Scientists use the periodic table to predict the properties and structures of elements. Elements in the same period have the same number of electron shells, whilst elements in the same group share similar chemical properties.

## Mark scheme (5 marks)

1. Magnesium has 2 electrons in its outer shell / highest energy level because it is in Group 2
2. Silicon has 4 electrons in its outer shell / highest energy level because it is in Group 4
3. Both magnesium and silicon are in Period 3, so both have 3 electron shells / 3 energy levels
4. Elements in the same group have similar chemical properties / react in similar ways
5. Across Period 3 from left to right, the number of outer electrons increases (from 1 to 8) / reactivity of metals decreases OR reactivity of non-metals increases / metallic character decreases

## Key terms

- [period](https://www.gradenine.co.uk/glossary/period)
- [group](https://www.gradenine.co.uk/glossary/group)

## Related

- [Revision notes for OCR A-Level Chemistry A (H432)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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