Ionization energy

A small amount of energy is supplied to an atom, then an electron may  be promoted to a higher energy .level, but if the amount of. energy supplied 
is sufficiently large the electron may be completely removed. The energy required to remove the most loosely bound electron from an isolated 
gaseous atom is called the ionization energy. 
Ionization energies are determined from spectra and are measured in kJ mo1- 1. It is possible to remove more than one electron from most 
atoms. The first ionization energy is the energy required to remove the first electron and convert M to M+; the second ionization energy is the energy 
required to remove the second election arid convert M+ to M2+; the third ionization energy converts M2+ to M3+, and so on.

The factors that influence the ionization energy are: 
1. The size of the atom. 
2. The charge on th,e nucleus. 
3. How effectively the inner electron shells screen the nuclear charge. 
4. The type of electron involved (s. p, d or/).

Reference Inorganic chemistry by Huhee