成链（catenation）是指同一种化学元素的原子经由连续的共价键互相连接形成长链状的分子。成链之形式在碳原子中最易出现，形成碳原子和碳原子之间相连的共价键。成链是自然界存在大量有机物质的原因，而有机化学实质上就是在研究碳利用这个性质所形成的化合物。然而，碳并非唯一拥有此性质的元素，其他主族元素也有形成长链的性质，如硅和硫。

化学元素能否形成长链，主要基于元素自身连接的键能，但也会受到位阻效应和电性因素的影响，包括：元素的电负性、混成分子轨域及元素之间形成不同共价键的能力。以碳元素为例，临近原子之间重叠的σ轨域可以足够强而可形成稳定的长链。以往认为其他元素很难形成长链，但现已发现许多元素都具有成链的分子结构。

Catenation is the linkage of atoms of the same element into longer chains. Catenation occurs most readily in carbon, which forms covalent bonds with other carbon atoms to form longer chains and structures. This is the reason for the presence of the vast number of organic compounds in nature. Carbon is most well known for its properties of catenation, with organic chemistry essentially being the study of catenated carbon structures (and known as catenae). However, carbon is by no means the only element capable of forming such catenae, and several other main group elements are capable of forming an expansive range of catenae, including silicon, sulfur and boron.