How do you find the valence shell number of a element?
For the representative, or main group elements, which are those in groups 1,2,13-18, or the "A" groups, which do not include the transition elements, the group number indicates the number of valence electrons.
The eight A groups have the same number of valence electrons as the group number.
If you use the numbering system of labeling each group from left to right as 1-18, groups 1 and 2 have one valence electron and two valence electrons respectively. For groups 13-18, the number of valence electrons is the group number minus 10.
Group 1/IA/1A: one valence electron
Group 2/IIA/2A: two valence electrons
Group 13/IIIA/3A: three valence electrons
Group 14/IVA/4A: four valence electrons
Group 15/VA/5A: five valence electrons
Group 16/VIA/6A: six valence electrons
Group 17/VIIA/7A: seven valence electrons
Group 18/VIIIA/8A: eight valence electrons