Naming Binary Compounds

The rules for naming compounds differ depending on what type of compound is being named. (An introduction to different types of compounds is available here).

Naming Ionic Compounds

Ionic compounds are named with the name of the metallic element first, followed by the name of the non-metallic element. The name of the non-metallic element is altered by changing the suffix of this element to -ide.

Formula	Name	Comments
NaCl	sodium chloride	common table salt
MgO	magnesium oxide	oxygen ® oxide (not oxygenide)
CaBr₂ Al₂O₃	calcium bromide aluminum oxide	For ionic compounds, name ignores number of each element.

For ionic compounds containing transition metals, a problem arises: The charge of the metal usually is not fixed. For example, both Fe⁺² and Fe⁺³ exist as stable ions. For ionic compounds containing these metals, the charge of the metal must first be determined based on the known charge of the anion. The name for these compounds will show this charge using roman numerals, which are placed in parentheses immediately following the name of the metal.

Formula	Name
NiCl₂	nickel(II) chloride
Fe₂O₃	iron(III) oxide
V₂O₅	vanadium(V) oxide

The most common mistake with this is to forget what the number in Roman numerals means. It does NOT indicate the number of atoms. It gives the (positive) charge of the transition metal. Iron(III) bromide contains a single iron atom with a +3 charge. It does not contain three iron atoms.

Naming Binary Covalent Compounds

Covalent compounds are named in a manner very similar to that used with ionic compounds, but with two differences. First, the element that is in the leftmost column is named first (no metal actually present). If both elements in the compound are in the same column, then whichever element is lower is named first. The second difference is that Greek prefixes are used to indicate the number of each element. (Note that the "mono-" prefix is optional and typically not used). Several examples are given below.

Greek Prefixes
Number	Prefix
1	mono-
2	di-
3	tri-
4	tetra-
5	penta-

Formula	Name	Comments
H₂O	dihydrogen oxide	Water. Hydrogen is considered a non-metal
CCl₄	carbon tetrachloride	A non-polar organic solvent.
P₂O₅	diphosphorus pentoxide	Last "a" in tetra and penta prefixed often dropped.
CO	carbon monoxide	"mono-" typically not given, but need to distinguish from CO₂
SO₂	sulfur dioxide	Sulfur is below Oxygen on periodic table.