covalent compound (also known as a molecular compound): a compound made of atoms joined by covalent bonds (shared electrons).
molecule: a compound made of atoms joined by covalent bonds.
Unlike ionic compounds, covalent compounds have names that give the chemical formula of the molecule.
Rules:
1. The first atom in the formula has a number prefix only if the molecule contains more than one of that atom.
2. The last atom in the formula always has a number prefix, whether or not there are more than one of that atom.
3. For huge numbers of atoms, the number prefixes combine in reverse order of place value. (Yes, it’s weird.)
Examples:
CO2 has 1 carbon atom (“carbon”) and 2 oxygen atoms (“di-oxide”), so its name is carbon dioxide.
P2O5 has 2 phosphorus atoms (“di-phosphorus”) and 5 oxygen atoms (“pent-oxide”), so its name is diphosphorus pentoxide.
N2O has 2 nitrogen atoms (“di-nitrogen”) and one oxygen atom (“mono-oxide,” which we elide to “monoxide”), so its name is dinitrogen monoxide.
H2O has 2 hydrogen atoms and one oxygen atom, but only a dork would call it “dihydrogen monoxide[*]” instead of “water”. The same goes for NH3, which is called “ammonia.”
[*] Someone has created a humorous website, http://www.dhmo.org , which attempts to make people aware of the dangers of dihydrogen monoxide, or “DHMO.”