The first ionisation energy is the energy required to remove one electron from each atom in one mole of atoms to form one mole of gaseous 1+ ions.
The second ionisation energy refers to the removal of the next mole of electrons from the mole of gaseous 1+ ions.
Influencing factors: Nuclear charge Atomic radius Electron shielding
Ionisation energy DECREASES down a group.
Ionisation energy INCREASES across a period.
*Decreases - down *Increases - across