电子亲合能

X⁻_(g) → X_(g) + e⁻ ΔH=E_ea

在固態物理學之中，對於一表面的電子親合能定義不同。

除以上定義（定義一）之外，電子親合能還有兩種定義方式：

定義二和定義一等價，是將一物質的原子或分子獲得一個電子，變成 -1 价离子時放出的能量。此定義下的電子親合能和定義一相同。不過此定義下，放出能量時电子亲和能為正，吸收能量時电子亲和能為負，正負號的使用和一般熱力學的定義恰好相反。

定義三，是將一物質的原子或分子獲得一個電子，變成 -1 价离子時的能量變化。放熱時數值為負，吸熱時數值為正。定義三的正負號使用和一般熱力學的定義相同。但其電子親合能恰為定義一的負值。(此定義為舊版IUPAC定義)

本文採用定義一的電子親合能。

並非所有的元素的電子親合能均為正，電子親合能為正表示其 -1 價的離子需吸收能量才能變為電中性的原子（早期的教科書寫有些元素，例如稀有氣體，其電子親合能為負，此說法並未被現代的化學家接受）。若其陰離子較不穩定，容易變成原子，則其電子親合能較低。元素中氯的電子親合能最高，汞和稀有氣體等元素的電子親合能都接近零。一般來說，非金屬的電子親合能都比金屬高。

总的来说，同一周期从左至右，價殼層電子递增，使得原子穩定性上昇，原子半径递减，对电子的吸引能力渐强，因而電子親合能递增；同族元素从上到下，因原子半径的增大，而且總電子數增加，原子穩定性下降，元素电负性值递减。 实际上，随核电荷数递增或同族元素从上到下，电子亲和能的变化并不单调。

带有电子亲合能数据的元素周期表

下列数据以kJ/mol为单位。带星号的元素在量子力学基態被认为有接近零的电子亲合能。

列表及参考资料

電子親合能 E_ea 的定義也可以延伸到分子。如苯和萘的電子親合能為負值，而蒽 、菲、芘的電子親合能為正值。電腦模擬實驗證實六氰基苯 C₆(CN)₆ 的電子親合能較富勒烯要高。

列表及参考资料

• Koopmans' theorem
• One-electron reduction
• 游離能
• 电负性
• 价电子

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• Electron affinity （页面存档备份，存于互联网档案馆）, IUPAC 金色書的定義
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