Ionization energy

In physics and chemistry, ionization energy (IE) (American English spelling), or ionisation energy (British English spelling) is the minimum energy required to remove the most loosely bound electron(s) (the valence electron(s)) of an isolated gaseous atom, positive ion, or molecule. The first ionization energy is quantitatively expressed as

X(g) + energy ⟶ X+(g) + e−

where X is any atom or molecule, X+ is the resultant ion when the original atom was stripped of a single electron, and e− is the removed electron. Ionization energy is positive for neutral atoms, meaning that the ionization is an endothermic process. Roughly speaking, the closer the outermost electrons are to the nucleus of the atom, the higher the atom's ionization energy.

In physics, ionization energy (IE) is usually expressed in electronvolts (eV) or joules (J). In chemistry, it is expressed as the energy to ionize a mole of atoms or molecules, usually as kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol).

Comparison of ionization energies of atoms in the periodic table reveals two periodic trends which follow the rules of Coulombic attraction:

Ionization energy generally increases from left to right within a given period (that is, row).

Ionization energy generally decreases from top to bottom in a given group (that is, column).

The latter trend results from the outer electron shell being progressively farther 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 the most loosely bound electron from the species having a positive charge of (n − 1).