Question
Define ionsiation enthalpy. State its units.
Solution
Ionisation enthalpy is the minimum amount of enthalpy required to remove most loosely held electron from an isolated gaseous atom in its ground state.
Atom (g) + Ionisation enthalpy
Positive ion (g) + electron (g) Na (g) + IE(496 kJ) 
Na+(g) + e-
It is expressed either in terms of electron volts per atom (eV/atom) or kilo joules per mole of atoms (kJ mol-1). The two units are related as 1eV per atom = 96.3 kJ mol-1
Atom (g) + Ionisation enthalpy
Positive ion (g) + electron (g) Na (g) + IE(496 kJ) Na+(g) + e-It is expressed either in terms of electron volts per atom (eV/atom) or kilo joules per mole of atoms (kJ mol-1). The two units are related as 1eV per atom = 96.3 kJ mol-1
