Chemistry - A Molecular Science

(Nora) #1

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.


© by

North

Carolina

State

University
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