An element category is a group of elements whose electrical conductivities are similar. The element categories are divided into metals , semi-metals and non-metals . The element categories are sometimes further subdivided according to their physical and chemical properties, for example in the case of metals into alkali metals , alkaline earth metals , earth metals and transition metals .
The discovery of the element categories was the first step that led to the development of the periodic table .
Some of the subordinate element categories correspond exactly to a group in the periodic table , but there are also element categories that do not include elements of the same group. For example, as a member of the first main group, hydrogen does not count among the alkali metals. Likewise one boron to the third main group, but not to the (earth) metals, but the semi-metals. The other main and subsidiary groups correspond to the subordinate element categories: the carbon group , the nitrogen group , the chalcogens , the halogens and the noble gases .