Chemistry - A Molecular Science

Chapter 4 The Ionic Bond

NAMING OXOANIONS* In those oxoanions ending in -

ate

, the central atom is in its highest oxidation state. For

example, nitrogen (a 5A nonmetal) is in a +5 oxidation state in the nitr

ate

ion (NO

1- 3

).

Oxoanions ending in -

ite

have one less oxygen atom than those ending in -

ate.

Removing

an oxygen

atom

from an oxoanion reduces the oxidation state of the central atom by two,

but it does not affect the charge on the ion. Thus, the nitr

ite

ion (NO

1- 2

), which has one less

oxygen atom than does the nitr

ate

ion, retains the same charge as the nitrate ion, but the

oxidation state of the nitrogen atom is reduced from +5 to +3.

The oxoanions of the Group 7A elements ar

e an exception because

, unlike the others,

they each form four oxoanions. Consequently, both prefixes and suffixes must be used (see Table 4.2). In the

per

chlor

ate

ion (ClO

1- 4

), the chlorine is in its highest oxidation state

(+7). The chlor

ate

ion (ClO

1- 3

) has one less oxygen, so the oxidation state of Cl is two less

or +5. The chlor

ite

ion

(ClO

1- 2

) has one less oxygen than chlorate, which lowers the

oxidation state of Cl to +3. Finally, the

hypo

chlor

ite

ion (ClO

1-) has one less oxygen than

chlorite, and the oxidation state of the Cl is

reduced to +1. Similarly, perbromate is BrO

1- 4

,

bromate is BrO

1- 3

, etc. Note, however, that fluorine is the most electronegative element, so

it never has a positive oxidation state. Con

sequently, fluorine forms no oxoanions.

Table 4.2

Prefixes and suffixes of the oxoanions formed by

the 7A nonmetals.

Prefix

Suffix

Oxidation state

of halogen

Formula of ion

Per- -ate

+7

XO

1-^4

-ate

+5

XO

1-^3

-ite

XO

1-^2

+3

Hypo- -ite

+1

XO

1-^

Table 4.3

Some common protonated oxoanions

HCO

1- 3

hydrogen carbonate or bicarbonate ion

HPO

2- 4

hydrogen phosphate ion

H^2

PO

1-^4

dihydrogen phosphate ion

HSO

1- 4

hydrogen sulfate ion or bisulfate ion

HSO

1- 3

hydrogen sulfite ion or bisulfite ion

* Recall that oxoanions are those polyatomic anions that consist of a

central atom surrounded by a oxygen atoms.

Oxoanions with charges of -2 or

-3 can be protonated (a proton, H

1+, can be added)

and still retain a negative charge. The resulti

ng ion is named by placing “hydrogen” (or

“dihydrogen” for two hydrogens) in front of the anion name. Thus, addition of H

1+ to a

phosphate ion, PO

3- 4

, results in the hydrogen phosphate ion, HPO

2- 4

, while the addition of

two H

1+ ions results in the dihydrogen phosphate ion, H

PO 2

1- 4

. It should be noted that the

prefix “bi” is sometimes used instead of “hydrogen”. Thus, HCO

1- 3

is the hydrogen

carbonate ion or the

bi

carbonate ion, and HSO

1- 4

is both the hydrogen sulfate ion and the

bi

sulfate ion. The use of “bi” is an older me

thod, but it is still quite common. The common

examples of protonated oxoanions are given in Table 4.3.

No changes or additions are made to polyatomic ion names in their compounds. Some

examples: NH

Cl is ammonium chloride; Na 4

CO 2

is sodium carbonate; NaHCO 3

is 3

sodium hydrogen carbonate, Co(ClO

) 43

is cobalt(III) perchlorate, and (NH

) 43

PO

is 4

ammonium phosphate.

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North

Carolina

State

University