Naming Ionic Compounds
When naming ionic compounds, the order of naming is metal first and then non-metal (cation then anion). The nonmetal then has its ending changed to -“ide”.
Ex. NaCl → Sodium Chloride (instead of chlorine)
Sometimes, compounds end in a polyatomic ion, in which case, you would not change the ending to “ide”.
Ex. NH₄HSO₄ → Ammonium Bisulfate
Some atoms can exhibit multiple charges. These are atoms in the transition metals section of the periodic table. Some notable atoms that can do this are iron (Fe), cobalt (Co), and copper (Cu). Because of this unique property, we must specify the charge of the transition metals using roman numerals.
Ex. Fe₂O₃ → Iron (III) Oxide. Iron has a +3 charge, so we write it as Iron (III) Oxide.
However, FeO also exists, written as Iron (II) Oxide. Without the roman numerals, they would both be called Iron Oxide, and we wouldn’t be able to distinguish them.
Writing Ionic Formulas from their Name
As we described in the “Octet Rule” section, you can determine an element’s charge using the periodic table. For transition metals, the compound name will provide a roman numeral describing the charge.
Once you know the charges of the ions, find the lowest whole number ratio that balances out the charges.
Ex. Magnesium nitride. Mg will have a +2 charge. Nitrogen will have a -3 charge. To balance the charges we need 3 Mg²⁺ (+6 charge), and 2 N³⁻ (-6) charge. The +6 and -6 charges cancel out. This gives us Mg₃N₂. Now, we could write Mg6N4 (+12 and -12 charges cancel out), but we want the lowest whole number subscripts, so we use Mg₃N₂.
Molecular Compounds
When writing the name of a molecular compound, write the elements in the same order as its chemical formula. The ending of the second element also becomes “-ide.”
For molecular compounds, you also need to change the prefixes to indicate how many atoms of each element are in the molecule.
Ex. SF₆ → Sulfur hexafluoride (“hexa” indicates that there are 6 fluorine atoms in SF₆).
(Again remember to change the ending of the second element to ‘-ide’).
List of prefixes ↴
Mono – one
Di – two
Tri – three
Tetra – four
Penta – five
Hexa – six
Only add a prefix to the first element if there are multiple atoms of that element (do not add “mono” to the first element).Ex. H₂O → Dihydrogen monoxide
Ex. SF₄ → Sulfur tetrachloride (it is not monosulfur tetrachloride)
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