Molecular and Ionic Compounds

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CHEM 0094 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.

• 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, notHgCl, 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.

• 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.

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.

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)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

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
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.

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:

1	mono-
2	di-
3	tri-
4	tetra-
5	penta-
6	hexa-
7	hepta-
8	octa-
9	nona-
10	deca-

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

Examples

SO2	sulfur dioxide
SO3	sulfur trioxide
N2O	dinitrogen monoxide
NO	nitrogen monoxide
NO2	nitrogen dioxide
N2O4	dinitrogen tetroxide
N2O5	dinitrogen 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 C_nH_2n+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.