在一般化學與原子物理學中，電子親合能（或電子親和勢、電子親和力，electron affinity，Eea）的定義是，將使一個電子脫離一個氣態的離子或分子所需耗費，或是釋出的能量。

　
: : X−(g) → X(g) + e− ΔH=Eea

在固態物理學之中，對於一表面的電子親合能定義不同。



其他定義

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 價離子時放出的能量。此定義下的電子親合能和定義一相同。不過此定義下，放出能量時電子親和能為正，吸收能量時電子親和能為負，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 價離子時的能量變化。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。



元素的電子親合能

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。

總的來說，同一週期從左至右，價殼層電子遞增，使得原子穩定性上昇，原子半徑遞減，對電子的吸引能力漸強，因而電子親合能遞增；同族元素從上到下，因原子半徑的增大，而且總電子數增加，原子穩定性下降，元素電負性值遞減。
實際上，隨核電荷數遞增或同族元素從上到下，電子親和能的變化並不單調。

電子親合能 (週期表)

下列數據以 kJ/mole 為單位。帶星號的元素在量子力學基態被認為有接近零的電子親合能。

列表及參考資料



分子的電子親合能

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。

列表及參考資料



參見


* Koopmans' theorem

* One-electron reduction

* 游離能

* 電負性

* 價電子



注釋



參考資料

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外部連結


* Electron affinity （頁面存檔備份，存於網際網路檔案館）, IUPAC 金色書的定義

* Article about Electron Affinity