Ionisation energy

First ionisation energy is the energy required to remove one mole of electrons from 1 mol of gaseous atoms to form one mole of gaseous 1+ ions.

Factors affecting 1st i.e.:

1. Atomic radius-the more shells there are, less attractive force between nucleus and electrons as they are further away so there's a lower i.e.
2. Electron shielding-  more inner shells and electrons are a barrier between nucleus and outermost electron, less nuclear attraction so lower i.e.
3. Nuclear charge- more protons, more nuclear charge so greater attractive force between protons and electrons meaning a higher I.e.

Na(g) -------> Na+ (g) + e-

Increases across periods as nuclear charge increases

decreases down a group as electron shielding and atomic radius increases