The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy (IE₁). The first ionization energy for an element, X, is the energy required to form a cation with +1 charge:

X_(g) ⟶ X⁺_(g) + e^–

IE₁X_(g) ⟶ X⁺_(g) + e⁻ IE₁

The energy required to remove the second most loosely bound electron is called the second ionization energy (IE₂).

X + (g) ⟶ X²⁺_(g) + e⁻ IE₂X⁺_(g) ⟶ X₂⁺_(g) + e⁻ IE₂

The energy required to remove the third electron is the third ionization energy, and so on. Energy is always required to remove electrons from atoms or ions, so ionization processes are endothermic, and IE values are always positive. For larger atoms, the most loosely bound electron is located farther from the nucleus, and so is easier to remove. Thus, as size (atomic radius) increases, the ionization energy should decrease. Relating this logic to what we have just learned about radii, we would expect first ionization energies to decrease down a group and to increase across a period.