Give the formulae of the compounds formed from the following sets of elements:
$(a)$ Calcium and fluorine
$(b)$ Hydrogen and sulphur
$(c)$ Nitrogen and hydrogen
$(d)$ Carbon and chlorine
$(e)$ Sodium and oxygen
$(f)$ Carbon and oxygen

(N/A) To determine the chemical formula,we use the valency of each element:
$(a)$ Calcium $(Ca^{2+})$ and Fluorine $(F^{-})$: The formula is $CaF_{2}$.
$(b)$ Hydrogen $(H^{+})$ and Sulphur $(S^{2-})$: The formula is $H_{2}S$.
$(c)$ Nitrogen $(N^{3-})$ and Hydrogen $(H^{+})$: The formula is $NH_{3}$.
$(d)$ Carbon $(C^{4+})$ and Chlorine $(Cl^{-})$: The formula is $CCl_{4}$.
$(e)$ Sodium $(Na^{+})$ and Oxygen $(O^{2-})$: The formula is $Na_{2}O$.
$(f)$ Carbon $(C^{4+})$ and Oxygen $(O^{2-})$: Carbon can form $CO$ (Carbon monoxide) and $CO_{2}$ (Carbon dioxide) depending on the conditions.