The energy needed to remove one or more electrons from a neutral atom to form a positively charged ion is a physical property that influences the chemical behavior of the atom. By definition, the first ionization energy of an element is the energy needed to remove the outermost, or highest energy, electron from a neutral atom in the gas phase.

Ionization energy is the amount of energy required to remove an electron from an atom (to form a cation), also called ionization potential.

Ionization energy is the amount of energy required to remove an electron from an atom (to form a cation), also called ionization potential.

It can be defined as being the energy required to remove the outermost electron from a gaseous atom.