Ionic Bonding (Part IV)

The Last Step: Adding Polyatomic Ions

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.

To this Point, All Ions Have Been Monatomic

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-

But, this Part with Polyatomic Ions is Added

Charges of Common Polyatomic Ions1+ 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-

A polyatomic ion is a fixed unit—it cannot be built onto or subtracted from. For example, SO4

2- cannot suddenly change to SO3

2-. 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.

The Only Thing Different About Polyatomic Ions is that in Formulas

They Must be Handled Specially

Barium phosphateCalcium carbonateAmmonium sulfideLead (II) chromate

Writing Formulas and Naming Compounds Follow the Same Logic as Before

• Ba3(PO4)2

• CaCO3

• (NH4)2S

• PbCrO4

The polyatomic cation (ammonium) and anionshave special names that must be found in the“Charges of Common Polyatomic Ions” table.

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.

The Counting of Atoms is More Complicated for Ionic Compounds

Containing Polyatomic Ions

Sodium Chloride, NaClBarium Sulfide, BaSSodium Phosphide, Na3P

Iron (III) Oxide, Fe2O3

Titanium (IV) Phosphide, Ti3P4

Lead (IV) Fluoride, PbF4

For Simple Compounds, the Subscript Applies to Only One Element

• 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

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

For IC’s with Polyatomic Ions, Each Element in the Parentheses is Multiplied by the

Outside Subscript: This is a lot like algebra!

• 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