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