How do you find the valence shell number of a element?
By knowing the number of electrons of an element.
For example, sodium has an electron number of 11. Hence, we can write out its electron configuration of 2.8.1.. Like this, we know that sodium has a valence electron of 1.
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