1. Ionic Bonding (Part IV)
The Last Step: Adding Polyatomic Ions

2. To this Point, All Ions Have Been Monatomic
Monatomic ions are ions that are formed from a single atom (the prefix mono- means one).
Polyatomic ions are ions that consist of two or more atoms bonded together by covalent bonds (the prefix poly- means more than one).
The combination of several atoms in a polyatomic ion has an overall charge that is either positive or negative. That, of course, makes it an ion.
Even with polyatomic ions, a positive ion is called a cation and a negative ion is called an anion.
As before, in an ionic compound, the total charge of the cations must be in perfect balance with the total charge of the anions.

3. The Monatomic Ions Table is Still Used
Oxidation Numbers of Monatomic Ions 1+ 2+ 3+ 4+
cesium, Cs+
copper (I), Cu+
gold (I), Au+
hydrogen, H+
lithium, Li+
potassium, K+
silver, Ag+
sodium, Na+
barium, Ba2+
beryllium, Be2+
cadmium, Cd2+
calcium, Ca2+
cobalt (II), Co2+
copper (II), Cu2+
iron (II), Fe2+
chromium (II), Cr2+
lead (II), Pb2+
magnesium, Mg2+
mercury (II), Hg2+
nickel (II), Ni2+
platinum (II), Pt2+
tin (II), Sn2+
zinc, Zn2+
manganese (II), Mn2+
aluminum, Al3+
bismuth (III), Bi3+
boron, B3+
cobalt (III), Co3+
gold (III), Au3+
iron (III), Fe3+
antimony (III), Sb3+
chromium (III), Cr3+
lead (IV), Pb4+
platinum (IV), Pt4+
silicon, Si4+
tin (IV), Sn4+
titanium (IV), Ti4+ 5+
niobium (V), Nb5+
vanadium (V), V5+ 1- 2- 3- 4-
bromide, Br-
chloride, Cl-
fluoride, F-
iodide, I-
oxide, O2-
sulfide, S2-
nitride, N3-
phosphide, P3-
carbide, C4-

4. But, this Part with Polyatomic Ions is Added
Charges of Common Polyatomic Ions 1+ 1- 2- 3-
ammonium, NH4+
acetate,
CH3COO-
azide, N3-
bromate,
BrO3-
chlorate, ClO3-
cyanide, CN-
formate,
HCOO-
bicarbonate,
HCO3-
hydroxide, OH-
hypochlorite,
ClO-
iodate, IO3-
nitrate, NO3-
nitrite, NO2-
perchlorate,
ClO4-
periodate, IO4-
carbonate, CO32-
chromate, CrO42-
dichromate,
Cr2O72-
sulfate, SO42-
sulfite, SO32-
tetraborate,
B4O72-
thiosulfate,
S2O32-
phosphate,
PO43-

5. Thus, aluminum sulfate is Al2(SO4)3.
The Only Thing Different About Polyatomic Ions is that in Formulas They Must be Handled Specially
A polyatomic ion is a fixed unit—it cannot be built onto or subtracted from. For example, SO42- cannot suddenly change to SO32-.
In formulas, multiples of polyatomic ions are treated with parentheses and a subscript outside the parentheses.
Thus, aluminum sulfate is Al2(SO4)3.
A single polyatomic ion does not need parentheses. So, barium sulfate is BaSO4.

6. Writing Formulas and Naming Compounds Follow the Same Logic as Before
Barium phosphate
Calcium carbonate
Ammonium sulfide
Lead (II) chromate
Ba3(PO4)2
CaCO3
(NH4)2S
PbCrO4
The polyatomic cation (ammonium) and anions have special names that must be found in the “Charges of Common Polyatomic Ions” table.

7. The Counting of Atoms is More Complicated for Ionic Compounds Containing Polyatomic Ions
As before the subscript applies to the species at its immediate left only
If that is a parenthetical unit, each element within the parentheses gets multiplied by the subscript outside the parentheses.
Let’s review the simple ones with which you have worked before.

8. For Simple Compounds, the Subscript Applies to Only One Element
Sodium Chloride, NaCl
Barium Sulfide, BaS
Sodium Phosphide, Na3P
Iron (III) Oxide, Fe2O3
Titanium (IV) Phosphide, Ti3P4
Lead (IV) Fluoride, PbF4
1 Na and 1 Cl
1 Ba and 1 S
3 Na and 1 P
2 Fe and 3 O
3 Ti and 4 P
1 Pb and 4 F

9. For IC’s with Polyatomic Ions, Each Element in the Parentheses is Multiplied by the Outside Subscript: This is a lot like algebra!
Sodium phosphate, Na3PO4
Ammonium carbonate, (NH4)2CO3
Aluminum dichromate, Al2(Cr2O7)3
Silicon formate, Si(HCOO)4
Bismuth azide, Bi(N3)3
Chromium (III) sulfate, Cr2(SO4)3
Gold (III) Perchlorate, Au(ClO4)3
3 Na, 1 P, 4 O
2 N, 8 H, 1 C, 3 O
2 Al, 6 Cr, 21 O
1 Si, 4 H, 4 C, 8 O
1 Bi and 9 N
2 Cr, 3 S, 12 O
1 Au, 3 Cl, 12 O