Periodic Trends

This lesson covers the trends across periods and down groups in the periodic table — as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 1: Key Concepts in Chemistry. You need to be able to describe and explain trends in reactivity, atomic radius, and character (metallic/non-metallic) in terms of electron structure.

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Trends Down a Group

When you move down a group in the periodic table, several properties change in a predictable way. These trends are caused by changes in electron structure — specifically, the number of electron shells increases.

Atomic Radius Increases Down a Group

• As you go down a group, atoms have more electron shells.
• Each additional shell is further from the nucleus, making the atom larger.
• For example, lithium (2, 1) has 2 shells, sodium (2, 8, 1) has 3 shells, and potassium (2, 8, 8, 1) has 4 shells.