C
Complicated
li
d Ionic
I i Compounds
C
d
Complication 1. Polyatomic anions
When an ionic compound dissolves in water the metal part of the
compound becomes a positively charged cation, while the
non-metal part becomes a negatively charged anion. The first
complication
li ti to
t look
l k att is
i polyatomic
l t i anions,
i
i.e.
i anions
i
that
th t
contain more than one atom. You have probably already heard
of some of the polyatomic anions,
anions things like phosphates,
phosphates
nitrates, or sulfates. To recognize these polyatomic anions you
g of the all
will have to memorize the names,, atoms,, and charges
the polyatomic anions listed on the next page.
Complicated Ionic
Polyatomic Anions to memorize
Acetate
Carbonate
Hydroxide
Nitrate
Nitrite
Phosphate
Sulfate
Sulfite
Hydrogen Sulfate
H d
Hydrogen
S
Sulfite
lfit
Ammonium
C2 H3 O2 CO3 2OHNO3NO2PO43SO42SO32HSO4HSO3NH4+
Complicated Ionic Compounds
Cations of unknown charge
Complication 2. Cations of unknown charge
When an ionic compound dissolves in water the metal part
of the compound becomes a positively charged cation. With
the simple ionic compounds you could memorize the charge of
certain
t i metals
t l because
b
they
th don’t
d ’t change
h
from
f
compoundd to
t
compound. For any other metal, however, the charge can change,
so you have to specify the charge of the metal in the name itself.
itself
You do this by giving the charge of the metal in the compound
p Chromium(VI)
( ) for
with a Roman numeral. For example
Chromium with a +6 charge.
The hard part is figuring out the charge from the formula.
Complicated Ionic
Cations of unknown charge
The
h net charge
h
off a compoundd is
i always
l
given
i
as a superscript
i in
i
the formula. Most compound have no net charge, so you usually
don’tt see a superscript in a molecular formula.
don
formula
The net charge of a compound comes from adding the net positive
charge of the cations to the net charge of the anions. Since most
compound have no net charge, the sum of the positively charged
cations equals the sum of the negatively charged anions.
At this point you should know the charges of all the anions
so all it takes is a little math to get the charge of the cations.
Complicated Ionic
Cations of unknown charge
Example: MnO2
Compound:
Charge on compound - 0 (no superscript)
Anion:
atom: O
name: oxide
number in compound: 2
charge: –2,
Net ion charge: 2 · (-2) = -4
Cation:
atom: Mn
name: manganese
charge : unknown!!
number in compound:1
Calculation of cation charge:
0(net) = X·(1)(cation charge) + 2 · (-2) (anion charge)
0 = X – 4; X = +4;
Compound name: Manganese(IV) oxide
(Roman numerals in name give charge of cation)
Complicated Ionic
Cations of unknown charge
Example: FeCl3
Compound:
Charge on compound - 0 (no superscript)
Anion:
atom: Cl
name: chloride
charge: –1,
number in compound: 3
Net ion charge: 3· (-1) = -3
Cation:
atom: Fe
name: iron
charge : unknown!!
Number in compound: 1
Calculation of cation charge:
0(net) = X· 1(cation charge) + 3· (-1) (anion charge)
0 = X – 3; X = +3;
Compound name: Iron(III) chloride
Complicated Ionic
Cations of unknown charge
Example: Cr2O3
Compound:
Charge on compound - 0 (no superscript)
Anion:
atom: O
name: oxide
number in compound: 3
charge: –2,
Net ion charge: 3 x (-2) = -6
Cation:
atom: Cr
name: chromium
charge : unknown!!
number in compound: 2
Calculation of cation charge:
0(net) = X· 2(cation charge) + 3· (-2) (anion charge)
0 = 2X – 6; 6 = 2X; 6/2=X; X= +3
Compound name: Chromium(III) oxide
Complicated Ionic
Cations of unknown charge
Practice examples
OsO4
FeCl2
Cu(NO3)2
PbCl4
HgBr
g 2
AgNO3
PbCO3
CuI
Complicated Ionic
Cations of unknown charge
Practice examples
OsO4
FeCl2
Cu(NO3)2
PbCl4
HgBr
g 2
AgNO3
PbCO3
CuI
Osmium(VIII) oxide
Iron(II) chloride
Copper(II) nitrate
Lead(IV) chloride
Mercury(II)
y( ) bromide
Silver(I) nitrate
Lead(II) carbonate
Copper(I) iodide