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.

Electron shells and periodic-table position

Electrons in an atom occupy shells around the nucleus. The arrangement of these electrons helps locate the element in the periodic table: its group is a vertical column, and its period is a horizontal row.

Group and outer-shell electrons

For elements in Groups I to VII, the group number equals the number of electrons in the outer shell. For example, an element in Group III has three outer-shell electrons.

Group VIII contains the noble gases, which have full outer electron shells. Helium’s only shell is full with 2 electrons. The other noble gases among the first 20 elements—neon and argon—each have 8 electrons in their outer shell.

Period and occupied shells

The period number equals the number of occupied electron shells: shells that contain electrons. An element with electrons in three shells is in Period 3.

For example, chlorine has the electron arrangement 2, 8, 7. It has 7 outer-shell electrons, so it is in Group VII. It has 3 occupied shells, so it is in Period 3.