Atomic Structure: Ionisation Energy

Ionisation energy refers to the energy needed to remove an electron.
Electrons are electrostatically attracted to the nucleus, resulting in an energy-consuming, endothermic process.
Ionisation energies are measured in J/mol.
It’s necessary to understand exact definitions - the first ionisation energy means the energy required to remove an electron from each atom in 1 mole of gaseous atoms to produce 1 mole of gaseous ions.