Question

Define ionization enthalpy. What are its units? What is the principle of its measurement?

Answer 1

Hint: Ionization potential is an endothermic process which is generally expressed by the term I or I.P. Endothermic reactions are those chemical reactions in which the reactants absorb heat energy from the surroundings to form products.

Complete answer:
Ionization energy can be described as the amount of energy needed to remove the most loosely bound electron i.e. electron present in the outermost electron from an isolated gaseous atom of an element in its lowest energy state or also known by ground state to produce a cation is known as ionization potential. Energy removed to the first electron is called first ionization energy and to remove second electron it is termed as second ionization energy and so on in their respective manner. 
The main principle of its measurement is it gives us an idea about the reactivity of chemical compounds. It can also be used to determine the strength of chemical bonds. It is measured either in units of electron volts or KJ/Mol .
Depending on the ionization of molecules which often leads to changes in molecular geometry, ionization energy can be either adiabatic ionization energy or vertical ionization energy.

Note:
The successive ionization energy can be defined as the energy required for the removal of the second electron from the ion which suggests that 1 mole of gas with 1^st ion having +1 charge further gives +2 charge atom by losing one more electron from its shell.