An element's group number tells you how many the element has.

An element's group number tells you how many valence electrons the element has.

Explanation:

In the periodic table, each element is placed in a specific location because of its atomic structure.

The elements are arranged in rows,  called a period. All of the elements in a period have the same number of atomic orbitals.

The elements are also arranged in vertical columns. Each column is called a group. The elements in each group have the same number of electrons in the outer orbital. These electrons are called as valence electrons.  Example: Group 1 elements have 1 valence electron each.

the number of valence electrons for each molecule or ion is shown beneath the structure. remember hydrogen will not have more than two electrons. this hydrogen is part of a covalent bond (sharing two electrons). a double bond here would cause hydrogen to share four electrons with phosphorus.