Ionization Energy of the Elements

What you need to know about ionization energy

Ionization energy increases moving left to right across the periodic table and decreases as you move down a group.
Ionization energy increases moving left to right across the periodic table and decreases as you move down a group. Duncan Walker / Getty Images

The ionization energy, or ionization potential, is the energy required to completely remove an electron from a gaseous atom or ion. The closer and more tightly bound an electron is to the nucleus, the more difficult it will be to remove, and the higher its ionization energy will be.

Units for Ionization Energy

Ionization energy is measured in electronvolts (eV). Sometimes the molar ionization energy is expressed, in J/mol.

First vs Subsequent Ionization Energies

The first ionization energy is the energy required to remove one electron from the parent atom. The second ionization energy is the energy required to remove a second valence electron from the univalent ion to form the divalent ion, and so on. Successive ionization energies increase. The second ionization energy is always greater than the first ionization energy.

Ionization Energy Trends in the Periodic Table

Ionization energies increase moving from left to right across a period (decreasing atomic radius). Ionization energy decreases moving down a group (increasing atomic radius).

Group I elements have low ionization energies because the loss of an electron forms a stable octet. It becomes harder to remove an electron as the atomic radius decreases because the electrons are generally closer to the nucleus, which is also more positively charged. The highest ionization energy value in a period is that of its noble gas.

Terms Related to Ionization Energy

The phrase "ionization energy" is used when discussing atoms or molecules in the gas phase. There are analogous terms for other systems.

Work Function - The work function is the minimum energy needed to remove an electron from the surface of a solid.

Electron Binding Energy - The electron binding energy is a more generic term for ionization energy of any chemical species.

It's often used to compare energy values needed to remove electrons from neutral atoms, atomic ions, and polyatomic ions.