Electron affinity (E_ea) is the energy change when a gaseous atom gains an electron to form a negative ion. For most elements this process releases energy and so is exothermic, because the added electron is attracted to the nucleus. Electron affinity generally becomes more negative (more energy released) across a period, paralleling the rise in effective nuclear charge.

电子亲和能(E_ea)是气态原子获得一个电子形成负离子时的能量变化。对大多数元素而言,该过程释放能量,因而是放热的,因为所加入的电子受到原子核的吸引。电子亲和能通常沿一个周期变得更负(释放更多能量),与有效核电荷的增大相一致。