Ionisation energy

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

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.

Generally, the closer the electrons are to the nucleus of the atom, the higher the atom's ionization energy.

The units for ionization energy are different in physics and chemistry. In physics, the unit is the amount of energy required to remove a single electron from a single atom or molecule: expressed as an electron volt. In chemistry, the units are the amount of energy it takes for all of the atoms in a mole of substance to lose one electron each: molar ionization energy or enthalpy, expressed as kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol).

Comparison of IEs of atoms in the periodic table reveals two patterns:

(The latter is due to the outer electron shell being progressively further away from the nucleus with the addition of one inner shell per row as one moves down the column.)

The nth ionization energy refers to the amount of energy required to remove an electron from the species with a charge of (n-1). For example, the first three ionization energies are defined as follows:

The term ionization potential is an older name for ionization energy, because the oldest method of measuring ionization energies was based on ionizing a sample and accelerating the electron removed using an electrostatic potential. However this term is now considered obsolete. Some factors affecting the ionization energy include:

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