Molecular and Ionic Compounds



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.
  • cation is a positively charged ion
  • An anion is a negatively charged ion.
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.
  • Metal  +  Nonmetal  —>  ionic compound (usually)
  • Metal  +  Polyatomic ion  —>  ionic compound (usually)
  • Nonmetal  +  Nonmetal  —>  covalent compound (usually)
  • Hydrogen  +  Nonmetal  —>  covalent compound (usually)

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)

GroupElementCationIon name
IAHH+hydrogen ion
LiLi+lithium ion
NaNa+sodium ion
KK+potassium ion
CsCs+cesium ion
IIAMgMg2+magnesium ion
CaCa2+calcium ion
SrSr2+strontium ion
BaBa2+barium ion
IIIAAlAl3+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.)
  • Many of these ions have common or trivial names formed from the stem of the element name (the Latin name in some cases) plus the ending -ic or -ous. (-ic endings go with the higher possible charge, -ous endings go with the lower possible charge).
  • The systematic names (also known as the Stock system) for these ions are derived by naming the metal first, followed in parentheses by the charge written in Roman numerals. For the metals below that typically form only one charge, it is not usually necessary to specify the charge in the compound name.
  • For example, iron can form two possible ions, 2+ and 3+. The Fe2+ ion is known as the ferrous ion (common) or the iron(II) ion (systematic); the Fe3+ ion is known as the ferric ion (common) or the iron(III) ion (systematic).
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 Hg2Cl2notHgCl, while mercury (II) chloride is HgCl2.)
Ions of Some Transition Metals and Post-Transition Metals (Groups IVA and VA)

MetalIonSystematic nameCommon name
CadmiumCd2+cadmium ion
ChromiumCr2+chromium(II) ionchromous ion
Cr3+chromium(III) ionchromic ion
CobaltCo2+cobalt(II) ioncobaltous ion
Co3+cobalt(III) ioncobaltic ion
CopperCu+copper(I) ioncuprous ion
Cu2+copper(II) ioncupric ion
GoldAu3+gold(III) ion
IronFe2+iron(II) ionferrous ion
Fe3+iron(III) ionferric ion
ManganeseMn2+manganese(II) ionmanganous ion
Mn3+manganese(III) ionmanganic ion
MercuryHg22+mercury(I) ionmercurous ion
Hg2+mercury(II) ionmercuric ion
NickelNi2+nickel(II) ionnickelous ion
SilverAg+silver ion
ZincZn2+zinc ion
TinSn2+tin(II) ionstannous ion
Sn4+tin(IV) ionstannic ion
LeadPb2+lead(II) ionplumbous ion
Pb4+lead(IV) ionplumbic ion
BismuthBi3+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)

GroupElementAnionIon name
IVACC4-carbide ion
SiSi4-silicide ion
VANN3-nitride ion
PP3-phosphide ion
AsAs3-arsenide ion
VIAOO2-oxide ion
SS2-sulfide ion
VIASeSe2-selenide ion
TeTe2-telluride ion
VIIAFF-fluoride ion
ClCl-chloride ion
BrBr-bromide ion
II-iodide ion
IAHH-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

C2H3O2-acetate (OAc-)
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.
  • Thio- implies replacing an oxygen atom with a sulfur atom:


  • Replacing the first element in the formula with another element from the same group gives a polyatomic ion with the same charge, and a similar name:

Group VIIAGroup VIA
Group VA*Group IVA
* But note that nitrogen does not follow this pattern (i.e., nitrate, NO3-)

  • Some nonmetals form a series of polyatomic ions with oxygen (all having the same charge): ClO-, hypochlorite; ClO2-, chlorite; ClO3-, chlorate; ClO4-, perchlorate.
    • The -ate forms (formula and charge) must be memorized.  In some cases, the -ate form has three oxygens, and in some cases four oxygens.  The charge is the same for the entire series.
    • The -ite form has one less oxygen that the -ate form.
    • The hypo- stem -ite form has two less oxygens than the -ate form.
    • The per- stem -ate form has one more oxygen than the -ate form.
    • The -ide form is the monatomic anion (see Main-Group Nonmetals)
    • The general rules for such series are summarized in the table below:

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


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 ¹ CaOH2 !


Nomenclature of Ionic and Covalent Compounds

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

Cl2Not binary (only one type of atom), but diatomic (two atoms)
BrClBinary (two different elements), and diatomic (two atoms)
H2OBinary (two different elements), but not diatomic (more than two atoms)
CH4Binary (two different elements), but not diatomic (more than two atoms)
CHCl3Neither 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.

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.

NaOHSodium hydroxide
Ca(NO3)2Calcium nitrate
K3PO4Potassium phosphate
(NH4)2SO4Ammonium sulfate
NH4FAmmonium fluoride
CaCO3Calcium carbonate
Mg(C2H3O2)2Magnesium acetate
Fe(OH)3Iron(III) hydroxide or ferrous hydroxide
Cr3(PO4)2 Chromium(II) phosphate
CrPO4Chromium(III) phosphate
NaHCO3Sodium 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
-idehydro- -ic + acid

ExampleCompound NameAcid name
HClO3hydrogen chloratechloric acid
H2SO4hydrogen sulfatesulfuric acid
HClO2hydrogen chloritechlorous acid
HClhydrogen chloridehydrochloric 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:

NaHSO4sodium hydrogen sulfate
NaH2PO4sodium dihydrogen phosphate
Na2HPO4sodium hydrogen phosphate
NaHCO3sodium 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.
  • The formula is written with the more electropositive element (the one further to the left on the periodic table) placed first, then the more electronegative element (the one further to the right on the periodic table).
[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.]
  • The first element in the formula is given the neutral element name, and the second one is named by replacing the ending of the neutral element name with -ide. A prefix is used in front of each element name to indicate how many atoms of that element are present:

  • If there is only one of the first element in the formula, the mono- prefix is dropped.

SO2sulfur dioxide
SO3sulfur trioxide
N2Odinitrogen monoxide
NOnitrogen monoxide
NO2nitrogen dioxide
N2O4dinitrogen tetroxide
N2O5dinitrogen 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.


(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


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.

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