ionization potential

The ionization energy (IE) is qualitatively defined as the amount of energy required to remove the most loosely bound electron of an isolated gaseous atom to form a cation. It is quantitatively expressed in symbols as:

It is also same as the 'ionization enthalpy' or 'ionisation potential'.

X + energy → X⁺ + e⁻  http://www.avogadro.co.uk/light/ionise.htm

where X is any atom or molecule capable of being ionized, X⁺ is that atom or molecule with an electron removed, and e⁻ is the removed electron. This is an endothermic process.