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MAIN General

Formulas and Nomenclature
of
Ionic and Covalent Compounds

Adapted from McMurry/Fay, section 2.10, p. 56-63
and the 1411 Lab Manual, p. 27-31. (References)

 

Contents:
Types of Compounds
Types of Ions:
     Main-Group Metals (Groups IA, IIA, and IIIA)
     Transition (B-group) and Post-Transition (Group IVA and VA) Metals
     Main-Group Nonmetals (Groups IVA, VA, VIA, and VIIA)
     Polyatomic Ions
Writing Formulas of Ionic Compounds
Nomenclature of Ionic and Covalent Compounds
     1.  Binary Ionic Compounds Containing a Metal and a Nonmetal
     2.  Ionic Compounds Containing a Metal and a Polyatomic Ion
     3.  Acids and Acid Salts
     4.  Binary Covalent Compounds Between Two Nonmetals
     5.  Hydrocarbons
Molecular Masses from Chemical Formulas
References

 

 

Types of Compounds

Ionic compounds are compounds composed of ions, charged particles that form when an atom (or group of atoms, in the case of polyatomic ions) gains or loses electrons.

Covalent or molecular compounds form when elements share electrons in a covalent bond to form molecules. Molecular compounds are electrically neutral.

Ionic compounds are (usually) formed when a metal reacts with a nonmetal (or a polyatomic ion).  Covalent compounds are formed when two nonmetals react with each other.  Since hydrogen is a nonmetal, binary compounds containing hydrogen are also usually covalent compounds.

 

 

 

Types of Ions:

Main-Group Metals (Groups IA, IIA, and IIIA)

Group IA, IIA, and IIIA metals tend to form cations by losing all of their outermost (valence) electrons. The charge on the cation is the same as the group number. The cation is given the same name as the neutral metal atom.

Ions of Some Main-Group Metals (Groups IA - IIIA)

Group Element Cation Ion name
IA H H+ hydrogen ion
  Li Li+ lithium ion
  Na Na+ sodium ion
  K K+ potassium ion
  Cs Cs+ cesium ion
IIA Mg Mg2+ magnesium ion
  Ca Ca2+ calcium ion
  Sr Sr2+ strontium ion
  Ba Ba2+ barium ion
IIIA Al Al3+ aluminum ion

 

 

Transition (B-group) and Post-Transition (Group IVA and VA) Metals

These elements usually form ionic compounds; many of them can form more than one cation. (The charges of the common transition metals must be memorized; Group IV and V metal cations tend to be either the group number, or the group number minus two.)

The mercury(I) cation is a special case; it consists of two Hg+ ions joined together, and so is always found as Hg22+. (Hence, mercury(I) chloride is Hg2Cl2, not HgCl, while mercury (II) chloride is HgCl2.)

Ions of Some Transition Metals and Post-Transition Metals (Groups IVA and VA)

Metal Ion Systematic name Common name
Cadmium Cd2+ cadmium ion  
Chromium Cr2+ chromium(II) ion chromous ion
  Cr3+ chromium(III) ion chromic ion
Cobalt Co2+ cobalt(II) ion cobaltous ion
  Co3+ cobalt(III) ion cobaltic ion
Copper Cu+ copper(I) ion cuprous ion
  Cu2+ copper(II) ion cupric ion
Gold Au3+ gold(III) ion  
Iron Fe2+ iron(II) ion ferrous ion
  Fe3+ iron(III) ion ferric ion
Manganese Mn2+ manganese(II) ion manganous ion
  Mn3+ manganese(III) ion manganic ion
Mercury Hg22+ mercury(I) ion mercurous ion
  Hg2+ mercury(II) ion mercuric ion
Nickel Ni2+ nickel(II) ion nickelous ion
Silver Ag+ silver ion  
Zinc Zn2+ zinc ion  
—————— ——— ———————— ————————
Tin Sn2+ tin(II) ion stannous ion
  Sn4+ tin(IV) ion stannic ion
Lead Pb2+ lead(II) ion plumbous ion
  Pb4+ lead(IV) ion plumbic ion
Bismuth Bi3+ bismuth(III) ion  
  Bi5+ bismuth(V) ion  

 

 

Main-Group Nonmetals (Groups IVA, VA, VIA, and VIIA)

Group IVA, VA, VIA, and VIIA nonmetals tend to form anions by gaining enough electrons to fill their valence shell with eight electrons. The charge on the anion is the group number minus eight. The anion is named by taking the element stem name and adding the ending -ide.

Ions of Some Nonmetals (Groups IVA - VIIA)

Group Element Anion Ion name
IVA C C4- carbide ion
  Si Si4- silicide ion
VA N N3- nitride ion
  P P3- phosphide ion
  As As3- arsenide ion
VIA O O2- oxide ion
  S S2- sulfide ion
VIA Se Se2- selenide ion
  Te Te2- telluride ion
VIIA F F- fluoride ion
  Cl Cl- chloride ion
  Br Br- bromide ion
  I I- iodide ion
IA H H- hydride ion

 

 

Polyatomic Ions

Polyatomic ions are ions that are composed of two or more atoms that are linked by covalent bonds, but that still have a net deficiency or surplus of electrons, resulting in an overall charge on the group. 

A metal plus a polyatomic ion yields an ionic compound.

Formulas and Names of Some Polyatomic Ions

Formula

Name

NH4+ ammonium
H3O+ hydronium
OH- hydroxide
CN- cyanide
O22- peroxide
N3- azide
NO2- nitrite
NO3- nitrate
ClO- hypochlorite
ClO2- chlorite
ClO3- chlorate
ClO4- perchlorate
MnO4- permanganate
C2H3O2- acetate (OAc-)
C2O42- oxalate
CO32- carbonate
OCN- cyanate
SCN- thiocyanate
S2O32- thiosulfate
CrO42- chromate
Cr2O72- dichromate
SO42- sulfate
SO32- sulfite
PO43- phosphate
PO43- monohydrogen phosphate
PO43- dihydrogen phosphate
HCO3- hydrogen carbonate (bicarbonate)
HSO4- hydrogen sulfate (bisulfate)
HSO3- hydrogen sulfite (bisulfite)

 

There are some regularities in the names of these polyatomic ions.

OCN- cyanate   SO42- sulfate
SCN- thiocyanate   S2O32- thiosulfate

 

Group VIIA   Group VIA
ClO3- chlorate   SO42- sulfate
BrO3- bromate   SeO42- selenate
IO3- iodate   TeO42- tellurate
Group VA*   Group IVA
PO43- phosphate   CO32- carbonate
AsO43- arsenate   SiO32- silicate

* But note that nitrogen does not follow this pattern (i.e., nitrate, NO3-)

 

Formula Name
XOny- stem + -ate
XOn-1y- stem + -ite
XOn-2y- hypo- + stem + -ite
XOn+1y- per- + stem + -ate
Xy- stem + -ide

Examples

SO42- sulfate
SO32- sulfite
SO22- hyposulfite
SO52- persulfate
S2- sulfide

 

 

Writing Formulas of Ionic Compounds

  1. The cation is written first, followed by the monatomic or polyatomic anion.
  2. The subscripts in the formula must produce an electrically neutral formula unit. (That is, the total amount of positive charge must equal the total amount of negative charge.)
  3. The subscripts should be the smallest set of whole numbers possible.
  4. If there is only one of a polyatomic ion in the formula, do not place parentheses around it; e.g., NaNO3, not Na(NO3). If there is more than one of a polyatomic ion in the formula, put the ion in parentheses, and place the subscript after the parentheses; e.g., Ca(OH)2, Ba3(PO4)2, etc.

Remember the Prime Directive in writing formulas:
Ca(OH)2 CaOH2 !

Examples

Cation Anion Formula

Na+

Cl-

NaCl

Ca2+

Br-

CaBr2

Na+

S2-

Na2S

Mg2+

O2-

MgO

Fe3+

O2-

Fe2O3

Na+

SO42-

Na2SO4

Mg2+

NO3-

Mg(NO3)2

NH4+

SO42-

(NH4)2SO4

 

 

 

Nomenclature of Ionic and Covalent Compounds

  1. Binary Ionic Compounds Containing a Metal and a Nonmetal. 

A binary compound is a compound formed from two different elements. There may or may not be more than one of each element. A diatomic compound (or diatomic molecule) contains two atoms, which may or may not be the same.

Cl2

Not binary (only one type of atom), but diatomic (two atoms)

BrCl

Binary (two different elements), and diatomic (two atoms)

H2O

Binary (two different elements), but not diatomic (more than two atoms)

CH4

Binary (two different elements), but not diatomic (more than two atoms)

CHCl3

Neither binary nor diatomic

Metals combine with nonmetals to give ionic compounds. When naming binary ionic compounds, name the cation first (specifying the charge, if necessary), then the nonmetal anion (element stem + -ide).

Do NOT use prefixes to indicate how many of each element is present; this information is implied in the name of the compound.

Examples

NaCl

 Sodium chloride

AlBr3

 Aluminum bromide

Ca3P2

 Calcium phosphide

SrI2

 Strontium iodide

FeCl2

 Iron(II) chloride or ferrous chloride
     The cation charge must be specified 
     since iron can form more than one charge.

 

 

  1. Ionic Compounds Containing a Metal and a Polyatomic Ion.

Metals combine with polyatomic ions to give ionic compounds. Name the cation first (specifying the charge, if necessary), then the polyatomic ion as listed in the table above (or as derived from the rules which were given).

Do NOT use prefixes to indicate how many of each element is present; this information is implied in the name of the compound.

Examples

NaOH

Sodium hydroxide

Ca(NO3)2

Calcium nitrate

K3PO4

Potassium phosphate

(NH4)2SO4

Ammonium sulfate

NH4F

Ammonium fluoride

CaCO3

Calcium carbonate

Mg(C2H3O2)2

Magnesium acetate

Fe(OH)3

Iron(III) hydroxide or ferrous hydroxide

Cr3(PO4)2

 Chromium(II) phosphate

CrPO4

Chromium(III) phosphate

NaHCO3

Sodium hydrogen carbonate or sodium bicarbonate

 

 

  1. Acids and Acid Salts.

Acids are compounds in which the "cation" is H+. (These are not really ionic compounds, but we'll get into that later.) These can be named as compounds as in the previous cases, e.g., HCl is "hydrogen chloride", but are more frequently given special "acid names" (especially when dissolved in water, which is most frequently the case.) The word "hydrogen" is omitted, the word "acid" is added to the end; the suffix is changed as shown below:

Compound name

Acid name

-ate

-ic + acid

 

 

-ite

-ous + acid

-ide

hydro- -ic + acid

Examples

Example

Compound Name

Acid name

HClO3

hydrogen chlorate

chloric acid

H2SO4

hydrogen sulfate

sulfuric acid

HClO2

hydrogen chlorite

chlorous acid

HCl

hydrogen chloride

hydrochloric acid

 

Acid salts are ionic compounds that still contain an acidic hydrogen, such as NaHSO4. In naming these salts, specify the number of acidic hydrogens in the salt. For instance:

Examples

NaHSO4

sodium hydrogen sulfate

NaH2PO4

sodium dihydrogen phosphate

Na2HPO4

sodium hydrogen phosphate

NaHCO3

sodium hydrogen carbonate or sodium bicarbonate

The prefix bi- implies an acidic hydrogen: thus, NaHCO3 is sodium bicarbonate (or sodium hydrogen carbonate); NaHSO3 is sodium bisulfite (or sodium hydrogen sulfite), etc.

 

 

  1. Binary Covalent Compounds Between Two Nonmetals.

Two nonmetals combine to form a covalent or molecular compound (i.e., one that is held together by covalent bonds which result from the sharing of electrons).

In many cases, two elements can combine in several different ways to make completely different compounds. (This cannot happen with ionic compounds, except in the cases of metals that can form more than one charge.) For instance, carbon can share electrons with one oxygen to make CO (carbon monoxide), or with two oxygens to make CO2 (carbon dioxide). For this reason, it is necessary to specify how many of each element is present within the compound.

[Important exception: when the compound contains oxygen and a halogen, the halogen is placed first. If both elements are in the same group, the one with the higher period number is named first.]

1

mono-

2

di-

3

tri-

4

tetra-

5

penta-

6

hexa-

7

hepta-

8

octa-

9

nona-

10

deca-

Examples

SO2

sulfur dioxide

SO3

sulfur trioxide

N2O

dinitrogen monoxide

NO

nitrogen monoxide

NO2

nitrogen dioxide

N2O4

dinitrogen tetroxide

N2O5

dinitrogen pentoxide

 

 

  1. Hydrocarbons.

Hydrocarbons contain only carbon and hydrogen, and are the simplest type of organic compound (a compound containing carbon). 

Alkanes contain only carbon-carbon single bonds, and are the simplest of the hydrocarbons. 

The simplest of the alkanes are the straight-chain alkanes, in which all of the carbon atoms are linked together in a line, with no branches. (They don't get simpler than that!) 

Alkanes have the general formula CnH2n+2, and are the constituents of several important fuels, such as natural gas and gasoline.

Organic chemistry has a completely different set of rules for nomenclature; straight-chain alkanes are named using a prefix plus the suffix -ane. Notice that after C4, the prefixes are the same as those listed above for binary covalent compounds.

CH4

methane

C2H6

ethane

C3H8

propane

C4H10

butane

C5H12

pentane

C6H14

hexane

C7H16

heptane

C8H18

octane

C9H20

nonane

C10H22

decane

 

(Because of the tremendous variety of possible organic compounds [over six million, and still counting], the rules for naming structures more complex than the staight-chain alkanes are much more elaborate than those that those we've seen so far, but those rules will be discussed when you take organic chemistry.) 

 

 

 

Molecular Masses from Chemical Formulas

The molecular mass, or molecular weight of a compound (measured in atomic mass units, amu) is obtained by adding up the atomic masses of all of the atoms present within a unit of the substance. 

For ionic compounds, the term formula mass or formula weight is used instead, since there aren't really any molecules present.

The molecular/formula mass is numerically equal to the mass of one mole of the substance.

For example, the molecular weight of water would be obtained by the following process:

Molecular mass of H2O = (2 x atomic mass of H) + (1 x atomic mass of O)
                                       = (2 x 1.00797) + (1 x 15.9994) amu
                                       = 18.02 amu

 

 

 

References

John McMurry and Robert C. Fay, Chemistry, 4th ed. Upper Saddle River, NJ:  Pearson/Prentice Hall, 2004, p. 56-63.

George E. Shankle & Harold W. Peterson, Laboratory Manual for Chemistry 1411.  University publication at Angelo State University, San Angelo, TX  76909, p. 27-31.