Like the formula of ionic compounds, the formula of most covalent compounds can be predicted by referring to the group in periodic table of the elements in the compound.
We can predict the formula of a covalent compound by referring to the valency of the elements in the compound, if we know which group in periodic table the element is located.
The valency of an atom is the number of electrons receive of release to achieve octet electrons arrangement.
For example, the electron arrangement of an oxygen atom is 2.6. To achieve octet electrons arrangement, the oxygen atom need to receive 2 electrons. Therefore, the valency of oxygen is 2.
Table below shows the group of the elements, the valency of the elements and the predicted formula of covalent compounds formed among the elements.

Element that combine				Formula of the covalent compound
Element X from	Valency	Element Y from	Valency
Group 15	3	Group 15	3	XY
Group 15	3	Group 16	2	X₂Y₃
Group 15	3	Group 17	1	XY₃
Group 16	2	Group 16	2	XY
Group 16	2	Group 17	1	XY₂
Group 17	1	Group 17	1	XY