Answer:
The valence electrons is the number of electrons in the outermost shell of an atom, it is the same as the group number;
Valency refers to the ability of an atom to form covalent bonds with another atom and corresponds to the group of the atom:
Group 1: valency +1
Group 2: valency +2
Group 3: valency +3
Group 4: valency +4 (also -4)
Group 5: valency -3
Group 6: valency -2
Group 7: valency -1
Group 8: valency 0