Ionization energy, also called ionization potential, is the energy required to remove an electron from a neutral atom. For example, converting Li into the Li+ cation requires ionization energy. Conversely, the energy released when a neutral atom gains an electron to form an anion is called electron affinity

Periodicity of Ionization Energy

The table lists ionization energies by atomic number. Across a period, ionization energy generally increases in a sawtooth pattern as atomic number and nuclear charge increase. Noble gases have the highest values because of their stable electron configurations. Group 1 has one valence electron, while Group 18 has a closed shell. This recurring pattern is the periodicity of ionization energy.

Electronegativity

Ionization energy and electron affinity can be used to estimate a relative value,Electronegativity, which describes an atom's ability to attract electrons and is roughly related to ionization energy. In simple terms, a higher value indicates a stronger tendency to gain electrons.

If an electron lies between two atoms, the more electronegative atom attracts it more strongly. Electronegativity is an important factor in chemical reactions. It generally increases toward the upper right of the periodic table.