Exception for molecular formulae: If the first element has only one atom, what should you do?

• A. If the first element has only one atom, do not add mono
• B. Always add mono
• C. Do not use prefixes at all
• D. Add mono for all elements

Answer: (A)

In naming covalent (molecular) compounds, prefixes show how many atoms of each element are present, but you do not use the prefix "mono" for the first element if there is only one atom. So if the first element appears once, you name it simply and apply prefixes to the second element as needed. For example, carbon monoxide uses carbon for the first element (no "mono" there) and monoxide for the second element, reflecting one oxygen. If the first element has two atoms, you would start with the appropriate prefix for it, such as di-, as in dinitrogen monoxide. The second element still receives its own prefix to show its quantity, including "mono-" when there is only one atom of that second element. This is why the correct approach is to omit "mono" for the first element when it is present as a single atom.