The amount of energy needed to steel an electron away from an atom.

Ionisation energy increases from left to right on the periodic table as the atoms are heavier and the nuclei have more charge. Metals on the left of the periodic table tend to loose electrons as they have a low ionisation energy.

Box notation can be used to show electrons in subshells.
Valence electrons are much easier to remove than core electrons

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Intermolecular forces

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