Question

Arrange the elements in decreasing order of the number of valence electrons
Br,S,Sb,Sr,F,Mg,I,Ca,Na,Kr,Xe,Bi,Ge,Ga,K

Answer 1

A valence electron is an electron in the outside shell. Br has 7, S has 6, Na has 1. Look at the group of the periodic table in which the electron is found. That often tells you the number of valence electrons. I don't know how your are supposed to handle a "decreasing order" when you run into Br with 7 and F with 7 since both are the same.
Here is a good site for you.
http://www.wikihow.com/Find-Valence-Electrons

Answer 2

rye