Here it is
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In this example X is a non metal
Y is also a non metal
Non metals and nonmetals the combined to form covalent molecules
they are going to share electrons
x needs only one more but Y is in need of three more
So Y is going to share its three electrons with three X atoms resulting in the formula
YX₃
Y can also share all 5 electrons with 5 X atoms to give  YX₅
Example -PCl₃ and PCl₅