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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.
A 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)
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.)
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 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 NN3- 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.
Thio- implies replacing an oxygen atom with a sulfur atom:
OCN-cyanate SO42- sulfate
SCN-thiocyanate S2O32- thiosulfate
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 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-)
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:
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 ¹ 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.
Cl2Not binary (only one type of atom), but diatomic (two atoms)
BrCl Binary (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.
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.
2. 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)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
3. 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
HClO3hydrogen chlorate chloric acid
H2SO4hydrogen sulfate sulfuric acid
HClO2hydrogen 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
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.
4. 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:
1mono-
2di-
3tri-
4tetra-
5penta-
6hexa-
7hepta-
8octa-
9nona-
10 deca-
If there is only one of the first element in the formula, the mono- prefix is dropped.
Examples
SO2sulfur dioxide
SO3sulfur trioxide
N2Odinitrogen monoxide
NO nitrogen monoxide
NO2nitrogen dioxide
N2O4dinitrogen tetroxide
N2O5dinitrogen pentoxide
5. 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.
CH4methane
C2H6ethane
C3H8propane
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.
12/5/23, 7:25 PM
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