Group number

The group number is the number of the vertical column an element sits in; for a main-group element it gives the number of valence electrons in its atoms. Malay: nombor kumpulan · Chinese: 族数.

This is one of the most useful shortcuts in the whole chapter. Once you know an element is in Group 1, you know at once that its atoms have one valence electron; Group 2 means two, and Groups 13 to 18 mean three through eight (Group 17 has seven, Group 18 has a full outer shell). You do not have to draw the electron arrangement first, the group number hands you the valence-electron count directly, and from there the ion it forms and its chemistry follow.

Example. Chlorine is in Group 17, so its atoms have seven valence electrons; it needs to gain just one electron to reach a stable octet, which is why it forms the Cl⁻ ion and is a reactive non-metal. Magnesium is in Group 2, so it has two valence electrons and loses them to form Mg²⁺. In both cases the group number told you the number of valence electrons, and the number of valence electrons told you the charge on the ion.

Confusion to avoid. Keep the group number apart from the period number: the group number counts valence electrons, while the period number counts occupied shells. It is also worth noting that this simple “group number equals valence electrons” rule applies to the main-group elements, not to the transition elements in the central block, whose behaviour is different because their electrons fill an inner shell as well. Modern tables number the columns 1 to 18, but older tables use Roman numerals such as Group I and Group VII, and you should recognise both.

In the Periodic Table of Elements chapter the group number is the quickest bridge from an element’s position to its chemistry, linking the table straight to bonding and reactivity.