Chemical similarity within a group

A group is a vertical column in the periodic table. Elements in the same group have the same number of electrons in their outer shell.

Outer-shell electrons determine how an atom takes part in chemical reactions: they are the electrons that can be lost, gained or shared. Because elements in a group have the same number of outer-shell electrons, they tend to react in similar ways and have similar chemical properties.

For example, lithium and sodium are both in Group 1 and each has one outer-shell electron. Each can lose that electron to form an ion with a charge of $$+1$$. Knowing that another element is in Group 1 therefore helps predict that it can behave similarly.

Elements in the same group are similar, but their reactions are not necessarily identical.