(a) What is the usual number of valence electrons and valency of group 18 elements of the periodic table?
(b) What happens to the number of valence electrons in the atoms of elements as we go down in a group of the periodic table?

(a) The usual number of valence electrons of group 18 in the periodic table is 8. Hence, its valency is zero.

(b) The number of valence electrons remains the same in the atom of elements as we move down the group of the periodic table.