Answer:
The number of valence electrons can be known from the group that the element is in (except the lanthanides, actinides and groups 3-12).
Explanation:
Group 1 has 1 valence electron. Group 2 has 2 valence electrons. Group 13 has 3 valence electrons. Group 14 has 4, group 15 has 5, group 16 has 6, group 17 has 7, and group 18 has 8.
Group 18 are the noble gases which are stable.
Group 17 are the halogens and react well with the alkali metals.
