Hey!

• Number of shells in an atom of the element tells about period number.
• Where as group number depends on the number of valence electrons. If the valence electrons are 1 , the group number equals the number of valence electrons :

1 Valence Electron - Group 1

2 Valence Electrons - Group 2

3 to 8 valence electrons - Add '10' to the valence electrons to get the group number of the element.

Example:

Chlorine (Cl) - Atomic Number 17

Electronic Configuration : 2 , 8 , 7

Period Number = Number of shells = 3

Group Number = Valence Electrons + “10” = 17

If it was magnesium then the group number would be 2 as UT has 2 valance electrons.

Hope it helped to clear your delimma.

Thank you