Ionization enthalpy is the minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom so to convert it into gaseous cation is called ionisation enthalpy.
It is measured in units of electron volts (eV) per atom or kilo calorie per mole or kilo joules per mole.
Factor affecting I.E includes the force of attraction between electrons and the nucleus, and the force of repulsion between electrons.
Principle of measurement-The ionization energy of a chemical element, expressed in joules or electron volts, is usually measured in an electric discharge tube in which a fast-moving electron generated by an electric current collides with a gaseous atom of the element, causing it to eject one of its electrons.
