The groups on the periodic table of elements, indicate what of the atom?

Columns (groups) are determined by the electron configuration of the atom; elements with the same number of electrons in a particular subshell fall into the same columns (e.g. oxygen and selenium are in the same column because they both have four electrons in the outermost p-subshell).

Each column is called a group.

Explanation:The elements in each group have the same number of electrons in the outer orbital. Those outer electrons are also called valence electrons. ... For example, nitrogen (N) has the atomic number seven.

i do not get it   at all