Reactions of Period 3 Elements

The reactions of Period 3 elements with oxygen and water reveal a clear trend: moving from left to right across the period, the elements become less metallic and their oxides change from strongly basic to amphoteric to acidic. This lesson covers the key reactions and the patterns they reveal.

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Reactions with Oxygen

All Period 3 elements (except argon) react with oxygen, though with varying vigour and producing different types of oxide.

Sodium + Oxygen

Sodium burns vigorously in air with a bright yellow flame:

4Na(s) + O₂(g) → 2Na₂O(s)

Sodium forms the ionic oxide Na₂O (sodium oxide), a white solid. In excess oxygen, sodium can also form the peroxide Na₂O₂.

The reaction is vigorous because sodium is a highly reactive Group 1 metal with a low ionisation energy — it readily loses its 3s¹ electron.

Magnesium + Oxygen

Magnesium burns with an intense white flame:

2Mg(s) + O₂(g) → 2MgO(s)

Magnesium oxide (MgO) is a white ionic solid with a very high melting point (2852 °C) due to strong ionic bonding between Mg²⁺ and O²⁻ ions.

Aluminium + Oxygen