Naming Compounds and Writing Their Formulas

0:00How do we name a compound and write its formula?

0:04First, is the compound ionic or covalent?

0:08In an ionic compound, a metal gives electrons to a non-metal, and both become ions: charged atoms.

0:16Name the metal first, then the non-metal ending in ide: sodium and chlorine make sodium chloride.

0:24Magnesium and oxygen make magnesium oxide.

0:29Magnesium loses two electrons to become two plus. Chlorine gains one to become one minus.

0:36Writing one of each is a common mistake: two plus and one minus do not cancel.

0:43A compound has no overall charge, so one magnesium ion needs two chloride ions: M g C l 2.

0:52The small 2 is a subscript: it counts the ions. A 1 is never written.

0:59Aluminum loses three electrons to become three plus. Oxygen gains two to become two minus.

1:07Two aluminum ions make six plus and three oxide ions make six minus, so aluminum oxide is A l 2 O 3.

1:17In a covalent compound, two non-metals share electrons, so no charges need balancing.

1:24Prefixes count the atoms: mono is one, di is two, tri is three, tetra is four.

1:33C O has one carbon and one oxygen: carbon monoxide.

1:38C O 2 has two oxygens: carbon dioxide.

1:43N 2 O 4 has two nitrogens and four oxygens: dinitrogen tetroxide.

1:50Mono is never used on the first element: not monocarbon monoxide.

1:56For a metal and a non-metal, balance the charges; for two non-metals, use prefixes to count the atoms.