Naming Binary Molecular Compounds

Binary molecular compounds are compounds that consist of exactly two nonmetal elements. Examples include HF, NO₂, and P₂O₅. Naming binary molecular compounds is really quite easy. The first element is given its element name; the second is given its root (hydr, bor, carb, ox, fluor, etc.) followed by ide.

Element Roots
H
hydrogen
	B	C	N	O	F
	boron	carbon	nitrogen	oxygen	fluorine
		Si	P	S	Cl
		silicon	phosphorus	sulfur	chlorine
			As	Se	Br
			arsenic	selenium	bromine
				Te	I
				tellurium	iodine

For example, HCl is hydrogen chloride, and H₂Se is hydrogen selenide.

Greek Prefixes Specify the Number of Atoms of Each Element

To be unambiguous (N₂O, NO, NO₂, and N₂O₄ all exist), we use Greek prefixes to explicitly indicate how many of each element are present. You need to memorize at least the first six Greek prefixes:

1=mono

2=di

3=tri

4=tetra

5=penta

6=hexa

Beyond that, it's simply a matter of remembering the following four rules to make the names a little more readable:

• Rule 1: Drop mono for the first element
• Rule 2: Drop all prefixes if the first element is H
• Rule 3: Drop the second o in mono prior to a vowel
• Rule 4: Drop the a in prefixes ending in a prior to a vowel

Examples include:

XnYm	n-preX-name m-preY-ide
N2S	dinitrogen monosulfide	(standard)
N2O3	dinitrogen trioxide	(standard)
CO2	carbon dioxide	Rule 1
HCl	hydrogen chloride	Rule 2
CO	carbon monoxide	Rule 3
N2O4	dinitrogen tetroxide	Rule 4

Summary

In summary, memorize the 15 nonmetal element names and symbols, memorize the first 6 prefixes, learn the four rules, and practice.

YOU CAN DO IT!