電子親合能

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，將單個電子結合到電中性的氣態原子或分子上所釋放的能量：
:X(g) + e− →  X−(g) + Eea
注意在此定義下，大多數元素原子的電子親和能為正數，即結合電子的過程是放熱的。這裡定義的電子親和能的正負號選取和一般熱力學的定義相反。
在固態物理學之中，對於一表面的電子親合能定義不同。

**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。

**帶有電子親合能數據的元素週期表**
下列數據以kJ/mol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 Eea 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
*遊離能
*電負性
*價電子

**注釋**

**參考資料**
以下許多參考資料的文章名稱會連結至參考資料的本文，不過需要訂閱或付費才能閱讀。(摘要)會連結到參考資料的摘要，不需付費即可閱讀。
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**外部鏈接**
 Electron affinity date=20070609215624 , IUPAC 金色書的定義- Article about Electron Affinity