The ionization energy of an element is defined as the amount of energy required to remove the most loosely bond electron form an atom of the element in the gas phase.

                M(g) + Ionization energy   →  M⁺(g) + e^-

One of the methods employed for measuring ionization energies is the electron impact (EI) method. Atoms of an element are bombarded with fast moving electrons which have sufficient energy to knock out an element of the atom with which they collide. The bombarding electrons may be given the appropriate amount of energy by accelerating at them through an electric potential which can be suitably varied. For example, hydrogen atom can lose an electron if the bombarding electron has a minimum energy of 2.18 X 10^-18 J.

              H(g) + e^-  →   H⁺(g) + 2e^-
 or         H(g) + 2.18 x 10^-18 J  →  H⁺(g) + e^-

Thus 2.18 x 10^-18 J is the ionization energy of one hydrogen atom. It is convenient to quote the total energy needed to remove an electron from each of the hydrogen atom in 1 mole of hydrogen atoms. That is,

2.18 x 10^-18J   X   6.022 x 10²³ atoms     = 1.31 x 10⁶ J mol^-1 =1310 KJ mol^-1
 1 atom                        1 mol

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