The first ionisation energy (I₁) is the energy required to remove the first, most loosely held electron from a neutral gaseous atom. It is the smallest of an element's successive ionisation energies. Its variation across the periodic table is widely used to interpret trends in reactivity and metallic character.

第一电离能(I₁)是从中性气态原子中移除第一个、束缚最松的电子所需的能量。它是某元素逐级电离能中最小的一个。其在周期表中的变化被广泛用于解释反应性和金属性的趋势。