Ionisation energy trends in the Periodic Table

5 minAS Level1.4.31.4.6

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can describe and explain the trends in first ionisation energy across periods 2 and 3 and down a group, including the dips at groups 13 and 16.

Real data, H to Ca. Saw-tooth shape: big drop at each new period, general rise across, two small dips per period.

Down a group, IE decreases

Group 1: Li 520, Na 496, K 419 kJ mol⁻¹. Group 18: He 2372, Ne 2081, Ar 1521.

The outer electron is in a shell further from the nucleus.
There are more inner shells, so more shielding.
These outweigh the increase in nuclear charge, so the outer electron is attracted less strongly.

Across a period, IE generally increases

Nuclear charge increases by one each step.
Electrons are added to the same shell, so shielding stays about the same.
The atomic radius decreases.
So the outer electron is attracted more strongly and is harder to remove.

The biggest drop comes straight after each noble gas (Ne → Na): the next electron starts a new shell, further out and shielded by a full inner shell.

Quick check

Multiple choiceQuestion 1 · Recall

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One-line answerQuestion 3 · Recall

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