Ionisation energy (I) is the energy required to remove an electron from a gaseous atom in its ground state. Because energy must be supplied to overcome the attraction of the nucleus, the process is endothermic. Ionisation energy generally increases across a period and decreases down a group, trends that reflect the effective nuclear charge.

电离能(I)是从处于基态的气态原子中移除一个电子所需的能量。由于必须供给能量以克服原子核的吸引,该过程是吸热的。电离能通常沿一个周期增大、沿一个族向下减小,这些趋势反映了有效核电荷。