不对称碳（也称为手性碳）是连有四种不同的原子或基团的碳原子。含有不对称碳的有机化合物的最大立体异构体数目可以按下面的方式计算：

例如，苹果酸的四个碳原子中有一个是不对称的。这个不对称碳原子连接有两个碳原子，一个氧原子和一个氢原子。尽管这个碳原子连接着的两个都是碳原子，但因为这两个碳原子所属的基团不同，因此这个碳原子仍然是不对称的。

有两个不对称碳原子的丁糖有2 = 4个立体异构体；

有三个不对称碳原子的戊糖有2 = 8个立体异构体；

有四个不对称碳原子的己醛糖有2 = 16个立体异构体；

以丁糖为例，4个立体异构体可以分为两组，每一组内的两个立体异构体互为镜像，它们分别称为同一种糖的左旋和右旋异构体。不能与自身的镜像重合的分子称为具有手性。

An asymmetric carbon atom (chiral carbon) is a carbon atom that is attached to four different types of atoms or groups of atoms. Le Bel-van't Hoff rule states that the number of stereoisomers of an organic compound is 2, where n represents the number of asymmetric carbon atoms (unless there is an internal plane of symmetry); a corollary of Le Bel and van't Hoff's simultaneously announced conclusions, in 1874, that the most probable orientation of the bonds of a carbon atom linked to four groups or atoms is toward the apexes of a tetrahedron, and that this accounted for all then-known phenomena of molecular asymmetry (which involved a carbon atom bearing four different atoms or groups). Knowing the number of asymmetric carbon atoms, one can calculate the maximum possible number of stereoisomers for any given molecule as follows: