Molecular compounds contain covalent bonds and generally form between nonmetals and nonmetals. When we name binary molecular compounds, we use prefixes to indicate the number of atoms of the two elements. Our prefixes go from mono for one atom to deca for 10 atoms. We do not use the prefix mono with our first element: if we have one atom of the first element, we just use its name without a prefix. For our second element, we change the ending to ide, similar to naming monatomic anions in ionic compounds. Let's practice naming binary molecular compounds. In our first example, we have one nitrogen atom and two oxygen atoms. The prefix for one is mono, but we do not use mono with our first element; the prefix for two is di. So, our compound is nitrogen dioxide. In our second example, we have two nitrogen atoms and one oxygen atom. We do use the prefix mono for our second element, so our compound is dinitrogen monoxide. Notice that we could drop the extra o from mono when used with oxide. So instead of monooxide we say monoxide. In our third example, our prefix for four atoms is tetra; our prefix for 10 atoms is deca. So our compound is tetraphosphorus decasulfide. In our last example, our prefix for five is penta. Here again, we can drop the a on penta and our compound is named dinitrogen pentoxide.