Ionization energy of an atom or molecule is the energy needed for a an electron to be removed from gaseous atoms or ions. It is also called Ionization potential.
A ----> A+ + 1e IE = ___ kJ/mol
It is often expressed in kilojoules per mole.
To convert the units from kilojoule per mole to joules per atom, we have to employ the Avogadro's number (6.022x10^23 atoms / mole)
Let's take the first ionization energy of sodium for example (496 kJ/mol).
First, we convert kJ to J (1000 J = 1 kJ) and then we convert the moles into atoms.
496 kJ/mol x ( 1000J/ 1kJ) x (1mol/ 6.022x10^23 atoms)
= 8.24x10^-19 joule per atom