5.9

Polyatomic Compounds

Polyatomic compounds are pure substance that involve combinations of polyatomic ions with metals, hydrogen, and other polyatomic ions. metal + poly.ion = hydrogen + poly.ion = poly.cmpd poly.ion + poly.ion = Polyatomic ions are groups of atoms that tend to stay together and carry an overall charge.

When a polyatomic compound is mixed with water the metal separates from the polyatomic ion.

Writing Formulas for Polyatomic Compounds

(Use handout: Common Polyatomic Ions and Compounds)

• Write the symbols

• Find the ionic charges

• Crisscross the charges (no need for the subscript 1)

• Reduce to lowest terms ***caution*** Examples: Sodium sulfate Lead (IV) carbonate Na

+1(SO4)

-2 Pb

+4(CO3)

-2

Na2(SO4) Pb2(CO3)4

Pb(CO3)2 (reduced)

5.9

Writing Names for Polyatomic Compounds

• Name of metal / polyatomic ion / hydrogen

• Name of polyatomic ion

Typically, it is the positively cation followed by the negatively charged polyatomic anion.

Oxyacids

Oxyacids are compounds formed when hydrogen combines whith polyatomic ions that contain oxygen.

Examples: Nitric acid HNO3

Nitrous acid HNO2 Chloric acid HClO3 Carbonic acid H2CO3 Sulfuric acid H2SO4 Sulfurous acid H2SO3 Phosphoric acid H3PO4

5.9

COMMON IONS

Symbols and Charges for Monoatomic Ions Fixed Charge

Symbol Name Symbol Name H+ hydrogen ion H- hydride

Li+ lithium ion F- fluoride

Na+ sodium ion Cl- chloride

K+ potassium ion Br- bromide

Rb+ rubidium ion I- iodide

Cs+ cesium ion

Ag+ silver ion O2- oxide

Be2+ beryllium ion S2- sulfide

Mg2+ magnesium ion Se2- selenide

Ca2+ calcium ion Te2- telluride

Sr2+ strontium ion

Ba2+ barium ion N3- nitride

Ra2+ radium ion P3- phosphide

Zn2+ zinc ion As3- arsenide

Al3+ aluminum ion

Symbols and Charges for Monoatomic Ions (Variable Charge)

Systematic name Common Systematic name

Common

Symbol (Stock system) name Symbol (Stock system) name Cu+ copper(I) cuprous Hg2

2+ mercury(I)

mercurous

Cu2+ copper(II) cupric Hg2+ mercury(II) mercuric

Fe2+ iron(II) ferrous Pb2+ lead(II) plumbous

Fe3+ iron(III) ferric Pb4+ lead(IV) plumbic

Sn2+ tin(II) stannous Co2+ cobalt(II) cobaltous

Sn4+ tin(IV) stannic Co3+ cobalt(III) cobaltic

Cr2+ chromium(II) chromous Au+ gold(I) aurous

Cr3+ chromium(III) chromic Au3+ gold(III) auric

Mn2+ manganese(II) manganous

Mn3+ manganese(III) manganic

Symbols and Charges for Polyatomic Ions

Formula Name Formula Name

NO3- nitrate ClO4

- perchlorate

NO2- nitrite ClO3

- chlorate

CrO42- chromate ClO2

- chlorite

Cr2O72- dichromate ClO

- hypochlorite

CN- cyanide IO4

- periodate

MnO4- permanganate IO3

- iodate

OH- hydroxide IO

- hypoiodite

O22- peroxide BrO3

- bromate

NH2- amide BrO

- hypobromite

CO32- carbonate HCO3

- hydrogen carbonate (bicarbonate)

SO42- sulfate HSO4

- hydrogen sulfate (bisulfate)

SO32- sulfite HSO3

- hydrogen sulfite (bisulfite)

C2O42- oxalate HC2O4

- hydrogen oxalate (binoxalate)

PO43- phosphate HPO4

2- hydrogen phosphate

PO33- phosphite H2PO4

- dihydrogen phosphate

S2O32- thiosulfate HS

- hydrogen sulfide

AsO43- arsenate BO3

3- borate

SeO42- selenate B4O7

2- tetraborate

SiO32- silicate SiF6

2- hexafluorosilicate

C4H4O62- tartrate

C2H3O2- acetate (an alternate way to write acetate is CH3COO

-)

There is one positive polyatomic ion. It is NH4+ and is called the ammonium ion.