Chemistry: What's in a Name? Covalent Nomenclature

What's in a Name? Covalent Nomenclature

Fortunately, it's much easier to name many covalent compounds than it is to name ionic compounds (see Ionic Compounds). Organic compounds have a separate naming system that we'll discuss, so if you've seen things like benzene and 3-methylhexane, don't worry about them just yet.

Naming Covalent Compounds from Formulas

Covalent compounds have two names. The following rules will enable us to name nonorganic covalent compounds with the greatest of ease:

You've Got Problems

Problem 1: Name the following covalent compounds:
(a) PCl3
(b) CO
(c) SF6

Rule 1

The first word is the name of the central atom. Since we don't yet know how to draw covalent compounds, how do we know which is the central atom? The central atom is usually the one that's least abundant in the compound. For example, the central atom of CF4 is carbon.

Rule 2

The second word is the name of the other atom in the compound, with "-ide" replacing the end of the element name. For example, at this point we would refer to CF4 as "carbon fluoride."

Rule 3

Prefixes will sometimes need to be added to the beginning of each word to indicate that more than one atom of the element is present. The most commonly used prefixes are shown in the following table:

Number of Atoms Prefix
1 mono- (use only for oxygen)
2 di
3 tri
4 tetra
5 penta
6 hexa
7 hepta
8 octa-
The Mole Says

If you examine the positions of the seven diatomic elements on the periodic table, six of them form the shape of a seven on the right side of the periodic table and hydro-gen is left all by itself in the top left. As a result, it's easy to remember the seven diatomic elements as being "the big seven and hydrogen."

In our example of CF4, carbon doesn't require a prefix (the only time we ever use a prefix for one atom is with oxygen), and fluorine will have the "tetra" prefix. As a result, CF4 is known as "carbon tetrafluoride."

You've Got Problems

Problem 2: Write the formulas of the following covalent compounds:
(a) hydrogen bromide
(b) silicon dioxide
(c) oxygen dichloride

Rule 4

A few very common molecules have names that don't follow this system. The most important include water (H2O), ammonia (NH3), and methane (CH4). If you name these compounds using the steps above, people will have no idea what you're talking about!

If you are presented with covalent compounds that consist of two or more atoms of the same element bonded together, the name of the molecule is the same as the name of the element. For example, Cl2 is simply called "chlorine."

Writing Formulas from Names

Writing formulas from names is the opposite of the process you just learned. For example, if you find that a chemical compound is called "nitrogen trichloride," this suggests that there is one nitrogen atom and three chlorine atoms, giving you a formula of NCl3. It's not too hard, and you won't have problems with this.

The only thing that might give you trouble are some of the elements. When most elements are named, you simply write the atomic symbol of the element. For example, "carbon" is written as C. However, some of the elements are diatomic, meaning that they naturally occur in molecules containing two bonded atoms. These elements include the halogens (F2, Cl2, Br2, I2), oxygen (O2), nitrogen (N2), and hydrogen (H2). As a result, if anybody tells you that they're doing a reaction with any of these seven elements, you'll need to remember the formulas above.

Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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