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Trends in First Ionisation Energies Down a Group

The first ionisation energy decreases down a group. Although the nuclear charge increases, its effect is outweighed by the increased radius and to a lesser extent the increased shielding. So the nuclear attraction for the outer electron decreases as you go down the group.

Group 1 ionisation energies in kJmol^-1

Element	1st I.E. kJmol^-1
Li	520.2
Na	495.8
K	418.8
Rb	403.0
Cs	375.7

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Specification: OCR Chemistry A (H032), AS-level, 3.1.1 Periodicity. Topic: Periodicity.

Key terms in this lesson

Ionisation: The loss or gain of electrons by an atom or molecule to form a charged ion.
First ionisation energy: The energy needed to remove one electron from each atom of one mole of atoms in their gaseous state to form one mole of gaseous +1 ions, under standard conditions.
Shielding: The repulsion of the electron to be removed by the electrons in the inner shells, which reduces nuclear attraction.
Group: A vertical column of the periodic table, containing elements with the same number of outer-shell electrons.

More in Periodicity

All 14 lessons in Periodicity · All OCR AS-level Chemistry topics