Three elements 'X', 'Y' and 'Z' have atomic numbers 7, 8, and 9 respectively. Write the formula of the compound formed when 'X' combines with 'Z'.

The elements X, Y and Z are nitrogen, oxygen and fluorine respectively. The valency of nitrogen is 3 and that of fluorine is 1. NF3 (by Criss cross method) XZ3 or NF3 is the formula of the compound formed when X combines with Z.