锕系元素以第Ⅲ族副族元素锕为首的一系列元素，是原子序数第89元素锕到第103元素铹，共15种放射性元素，在周期表中占有一个特殊位置。

锕系元素的名称是因为3族元素锕，有时也会符号An表示锕系元素。锕系元素绝大部份是f区元素，最高能量的电子是在5f电子层，锕系元素只有铹是d区元素。镧系元素中大部份也一様是f区元素，不过相较起来，锕系元素的化合价有较多的变化。

锕系元素原子基态的电子构型是5f6d7s，这些元素的核外电子分为7层，最外层都是2个电子，次外层多数为8个电子（个别为9或10个电子），从镤到锘电子填入第5层，使第5层电子数从18个增加到32个。

The actinide /ˈæktᵻnaɪd/ or actinoid /ˈæktᵻnɔɪd/ (IUPAC nomenclature) series encompasses the 15 metallic chemical elements with atomic numbers from****to 103, actinium through lawrencium.

The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide. All but one of the actinides are f-block elements, corresponding to the filling of the 5f electron shell; lawrencium, a d-block element, is also generally considered an actinide. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence. They all have very large atomic and ionic radii and exhibit an unusually large range of physical properties. While actinium and the late actinides (from americium onwards) behave similarly to the lanthanides, the elements thorium through neptunium are much more similar to transition metals in their chemistry.