Chemistry - A Molecular Science

5.3

NAMING BINARY COVALENT COMPOUNDS

Table 5.2

Prefixes used to indicate the number of atoms of

each element present in the formula of covalent compounds Number Prefix

Example

1 mono CO carbon monoxide 2 di

CO

carbon dioxide 2

3 tri

SO

sulfur trioxide 3

4 tetra CCl

carbon tetrachloride 4

5 penta PF

phosphorus pentafluoride 5

6 hexa SF

sulfur hexafluoride 6

7 hepta Cl

O 2

dichlorine hept(a)oxide 7

†^

8 octa * 9 nona * 10 deca *

* These prefixes are common in naming both organic and

inorganic compounds, but there are few examples of binary compounds of the type considered here.

† The final vowel in the prefix is usually omitted when the

first letter of the element begins with a vowel. Thus, CO is carbon monoxide, not carbon monooxide, and Cl

O 2

is a 7

heptoxide rather than a heptaoxide.

Binary compounds

are compounds that consist of only two elements; they are named by

giving the name of the less electronegative atom followed by the name of the more electronegative atom with its ending changed to -

ide

. Greek prefixes (Table 5.2) are used

to indicate the number of each type of atom in the molecule except that the prefix “mono” is not used to denote a single atom if it is

referring to the first element in the compound.

Thus, CO is carbon monoxide

†, not monocarbon monoxide. Hydrogen always has a +1

oxidation state when it is bound to nonmetals, so the formula of its binary compounds is not ambiguous. Therefore, prefixes are frequently omitted. For example, H

S is commonly 2

called hydrogen sulfide rather than dihydrogen sulfide. Finally, many binary covalent molecules have common names, such as water (H

O), ammonia (NH 2

), nitric oxide (NO), 3

and nitrous oxide (N

O), which is frequently referred to as laughing gas. 2

By convention,

the less electronegative element is

written first in the formula

(carbon

dioxide is written CO

, not O 2

C because O is more electronegative than C). Exceptions do 2

occur if one of the elements is hydrogen because many hydrogen containing compounds are acids, and, by convention, hydrogens at the beginning of the formula are considered to be acidic hydrogens. Hydrogen sulfide is written H

S because it is acidic, but ammonia is 2

written NH

because it is not acidic. 3

Example 5.2 a) Name the following compounds:

H^2

Se:

Two H atoms would ordinarily require a

prefix of “di”, and the name would be

dihydrogen selenide. However, there is only one compound formed between H and Se, so it is frequently called hydrogen selenide.

N^2

O^3

:

Two N atoms and three O atoms, so the name is dinitrogen trioxide.

SnCl

: 4

It is tempting to name this compound tin tetrachloride, and it is sometimes called that. However, tin is a metal, so the compound should be named using the rules given in Chapter 4. The accepted name is tin(IV) chloride.

ClF

: 3

Three fluorine atoms, so this

is chlorine trifluoride.

b) What is the formula of disulfur decafluoride?

The prefix di means two and the prefix

deca is ten, so the compound is S

F 2

. 10

Chapter 5 The Covalent Bond

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