化学序是指一组化学元素，其中各元素的物理和化学特征从位于该串行一端的元素渐进地变化到位于另一端的元素。

早在元素周期表出现之前就已发现了化学序的规律，人们试图通过它来根据各元素的化学性质将元素分组进行组织。

许多化学序与周期表中的分族精确地对应。这并非巧合，分组所依据的物理性质其实同样是由导致这些元素处于周期表中同一分族中的原子轨道构型所决定的.

周期表中的化学序包括：

In chemistry, a group (also known as a family) is a column of elements in the periodic table of the chemical elements. There are 18 numbered groups in the periodic table, but the f-block columns (between groups 2 and 3) are not numbered. The elements in a group have similar physical or chemical characteristics of the outermost electron shells of their atoms (i.e., the same core charge), as most chemical properties are dominated by the orbital location of the outermost electron. There are three systems of group numbering. The modern numbering group 1 to group 18 is recommended by the International Union of Pure and Applied Chemistry (IUPAC). It replaces two older naming schemes that were mutually confusing. Also, groups may be identified by their topmost element or have a specific name. For example, group 16 is variously described as oxygen group and chalcogen.