Chapter 5
Molecules and
Compounds
Homework
 Assigned Problems (odd numbers only)
 Required
 “Problems” 23-79 (odd)
 “Cumulative Problems” 81-97 (odd)
 Highlight Problems
 “Highlight Problems” 99, 101 (odd)
Compounds Display Constant Composition
 Compounds are pure substances consisting of
two or more elements held together by a
chemical bond
 A compound can be broken into its elements by
a chemical process
 The percentage of each element in a specific
compound does not vary
 The Law of Definite Proportions (Constant
Composition): In a pure compound, the elements are
always present in the same definite proportions by
mass
Compounds Display Constant Composition
Example:
 Two samples of ammonia gas with different sample
masses
 Upon decomposition, the mass ratios of nitrogen to
hydrogen are the same
Sample 1:
1.513 g
 4.63
0.327 g
Sample 2:
1.644 g
 4.62
0.356 g
Chemical Formulas:
How to Represent Compounds
 Chemical formulas are a concise way of stating
a specific chemical compound’s composition
 The notation contains the symbols of the
elements present and the subscripts to indicate
the number of atoms per element in a structural
unit of the compound
Aspirin’s chemical formula is C9H8O4
Element symbols
21 atoms are present:
9 carbon atoms, 8 hydrogen atoms, and 4 oxygen atoms
Ratio of C to H to O is 9:8:4
subscripts
Chemical Formulas:
How to Represent Compounds
 All pure samples of a compound have the same
composition
 If only one atom is present in a molecule, the
subscript “1” is omitted
 Follow the capitalization rules for elemental
symbols
Co is the symbol for cobalt
CO is the formula for carbon monoxide
 When writing formulas, list the metal or most
metallic element first
sodium chloride NaCl
carbon dioxide CO2
not
not
ClNa
O 2C
Chemical Formulas:
How to Represent Compounds
 If a chemical formula contains groups of atoms in
parentheses, the subscript following the parentheses
indicates the number polyatomic units in the formula
 Number of atoms is determined by multiplying the
subscript outside the parenthesis by the subscript for
every atom inside the parenthesis
 Composition is 1 Mg atom, 2 N atoms, 6 O atoms
Polyatomic ion
A Molecular View of
Elements and Compounds
 Pure substances
 Elements
 Atomic
 Molecular
 Compounds
 Ionic
 Molecular
Elements and Compounds
 Elements are substances that cannot be decomposed into simpler
substances
 Each element is composed of only one type of atom
 Most elements are atomic elements and exist as single atoms
(monatomic)
 Molecular elements are found naturally as two or more of the same
type of atom bound together (diatomic)
Elements and Compounds
 A compound can be decomposed into two or more
simpler substances
 The ultimate breakdown products are elements
 Two types of compounds
 Molecular
 Ionic
Molecular Compounds
 A molecule is a group of two or more atoms
bound together as a unit
 A molecular compound involves a bond between
two or more nonmetals
 Bonds occur between different or identical
atoms
 The individual molecule is the smallest particle
that can exist as a stable, independent unit of
the compound
Molecular (Covalent) Compounds
 Molecular compounds are compounds composed of
molecules that exist as the individual structural unit
 The simplest molecule contains two atoms (diatomic).
They can be the same or different atoms
 Next in complexity are triatomic and tetraatomic
O2
O3
oxygen
ozone
A diatomic molecule
A triatomic molecule
CO
H 2O
carbon monooxide
water
A diatomic molecule
A triatomic molecule
Ionic Compounds
 Ionic compounds are compounds which
are held together by the attraction of
positive and negative ions
 Solid crystals formed by a very ordered
packing of oppositely charged ions
 Most ionic compounds are composed of a
metal and a nonmetal
 High melting temperatures
Ionic Compounds
 Solids
 They do not exist as single




molecules
The chemical formula
represents the simplest ratio
that these atoms combine
together
Held strongly together, these
ions are packed together into a
“lattice”
Each Na+ ion is surrounded on
each side by Cl- ions
Each Cl- ion is surrounded on
each side by Na+ ions
Cl–
Na +
Cl–
Cl–
Na +
Cl–
Na +
Cl–
Na +
Cl–
Na +
Na +
Fig4_21a
Formula unit = NaCl
Ionic Compounds
vs. Molecular Compounds
 For molecular
compounds, the
chemical formulas
give the composition
of the molecules
 For ionic compounds,
the chemical formulas
give the ion ratio
within the compound
Writing Formulas for Ionic Compounds
(Charge Balance in Ionic Compounds)
 Binary ionic compounds are composed of
only two elements (metal and nonmetal)
 The symbol of the cation always
precedes the symbol of the anion
 The sum of the positive charges (cation)
must equal the sum of the negative
charges (anion)
 Net charge is zero (charge-neutral)
 Subscripts written as whole numbers
indicate the number of each ion in the
formula unit
Writing Formulas for Ionic Compounds
Subscripts in Formulas
 Sodium Chloride
 Formed from
sodium and
chlorine atoms
 An ionic bond
forms consisting
of a sodium ion
(+ charge) and a
chloride ion
(- charge)
 Each sodium loses
one electron to
achieve an octet
 Each chlorine atom
gains one electron
to achieve an octet
 Formula is NaCl
Writing Formulas for Ionic Compounds
Subscripts in Formulas
 Magnesium
Chloride
 Formed from
magnesium and
two chlorines
 An ionic bond
forms consisting
of a magnesium
ion (2+ charge)
and
two chloride ions
(- charge each)
 Each magnesium
loses two electrons
to achieve an octet
 Each chlorine atom
gains one electron
to achieve an octet
 Formula is MgCl2
Writing Ionic Formulas for
Ionic Charges
 Subscripts in a formula represent the number of
positive and negative ions
 Write the formula for the ionic compound containing
Na+ and N3-
Na+
N
••
• • 3-
N
••
••
••
•
Na•
••
••
Na+
••
Na•
••
•
Na•
Gains
Each loses 1e-
3e-
•
•
•
Na+
Net charge: 3(1+) + 1(3-)=0
Formula: Na3N
Naming Ionic Compounds
For ionic compounds containing two elements:
 Compounds containing a metal and a
nonmetal are called binary ionic compounds
 Two types of metals:
 Single Cation Metals: Form one positive ion
 Multiple Cation Metals: Form more than one
positive ion
 The systematic naming uses the name of the
cation first, followed by the name of the anion
 Subscripts in the formula are not included in
the name
Types of Metal Ions
 Single Cation Metals (Type I)
 Form only one type of ion (one possible
charge)
 Main group metals in groups IA, IIA, and
some IIIA
 e.g. Sodium only forms one ion (Na+) in
chemical reactions
 Determine charge by position on the
periodic table (also see table 5.3 on
page 136)
Types of Metal Ions
 Single Cation Metals (Type I)
 Form only one type of ion (one possible charge)
 Main group metals in groups IA, IIA, and some IIIA
 e.g. Sodium only forms one ion (Na+) in chemical
reactions
 Determine charge by position on the periodic table
(also see table 5.3 on page 136)
Types of Metal Ions
 Multiple Cation Metals (Type II)
 Form two or more types of ions (variable
possible charge)
 Transition metals in groups 3B to 12B,
and some 4A and 5A
 For example, iron forms two ions (Fe2+
and Fe3+) in chemical reactions
 Determine charge by the “stock system”
for naming ions
 The metal name followed by a Roman
numeral in parentheses to indicate its
charge (see table 5.4 on page 136)
Types of Metal Ions
 Multiple Cation Metals (Type II)
 Form two or more types of ions (variable possible charge)
 Transition metals in groups 3B to 12B, and some 4A and 5A
 For example, iron forms two ions (Fe2+ and Fe3+) in chemical
reactions
 Determine charge by the “stock system” for naming ions
 The metal name followed by a Roman numeral in parentheses to
indicate its charge (see table 5.4 on page 136)
Naming Type I Binary Ionic Compounds
 Compounds which contain a positively
charged metallic ion and a negatively charged
nonmetallic ion
 Name metal cation first, name nonmetal anion
second
 Single metal cation name is the metal name
only, drop the word “ion”
 Nonmetal anion named by changing the
ending on the nonmetal name to -ide
Name of metal__base name of nonmetal + -ide
Naming Type I Ionic Compounds
 NaI
KCl
Potassium Chloride
 Sodium Iodide
Na3P
 CaF2
 Calcium Fluoride Sodium Phosphide
Rb2S
 Li2O
Rubidium Sulfide
 Lithium Oxide
Mg3N2
 AgCl
Magnesium Nitride
 Silver Chloride
Naming Type II Binary Ionic Compounds
Multiple Cation Metal Compounds
 Metal listed first in formula & name (same
order as for Type I compounds)
 Determine metal cation charge from the
anion charge
 Common multiple cations in Table 5.4,
page 136
Naming Type II Binary Ionic Compounds
Multiple Cation Metal Compounds
 Use the metal name (cation) first, followed
by a Roman numeral in parentheses to
indicate its charge
 Nonmetal anion named by changing the
ending on the nonmetal name to -ide
cation
anion
Name of metal (charge of metal)______base name of nonmetal + -ide
Naming Type II Binary Ionic Compounds
 FeI3
 1(?)+3(-1)=0
 Iron (III) Iodide
 Cu2O
 2(?)+1(-2)=0
 Copper (I) Oxide
 SnBr2
 1(?)+2(-1)=0
 Tin (II) Bromide
Naming Type II Binary Ionic Compounds
Determining the Charge of the Cation from the Anion
Determine the charge of Cu in Cu2O
Write the name of the compound
1)Determine the charge of the cation from
the anion
 Cu2O - the nonmetal anion is O, since it is
in Group 6A, its charge is 2 Since there are 2 Cu ions in the formula
and the total positive charge is 2+, divide
by the number of cations so each Cu has
a 1+ charge
Naming Type II Binary Ionic Compounds
Determining the Charge of the Cation from the Anion
2) Name the cation by its element name and
use a Roman numeral in parenthesis to
indicate its charge
 Copper (I)
3) Name the anion by changing the last part of
its element name to –ide
 Oxygen
oxide
4) Write the name of the cation first and the
name of the anion second
copper (I) oxide
Naming Type II Binary Ionic Compounds
Determining the Charge of the Cation from the Anion
• SnI4
• 1(?)+4(-1)=0
• Tin (IV) iodide
• HgO
• 1(?)+1(-2)=0
• Mercury (II) Oxide
• MnCl2
• 1(?)+2(-1)=0
• Manganese (II) Chloride
Writing Formulas from the Name of
an Ionic Compound

Usually involves a metal and a nonmetal
1) Identify the cation and the anion
2) Balance the charges to write the formula
 If it is a multiple cation metal, the Roman
numeral determines the charge of the
cation
3) When writing the formula, take the name of
the cation first, followed by the name of the
anion
Writing Formulas from the Name of
an Ionic Compound
 Compound name is lithium chloride
 Li+ and Cl- are the ions
 Balance the charges


Li
Cl
1(1)  1 (1)  0
 Write the formula
 LiCl is the formula using the
subscripts from the charge balance
Writing Formulas from the Name of
an Ionic Compound
 Compound name is iron (III) oxide
 Fe3+ and O2- are the ions
 Balance the charges
Fe 3
O2 
2 (3)  3 (2)  0
 Write the formula
 Fe2O3 is the formula using the
subscripts from the charge balance
Polyatomic Ions
 Polyatomic ions are a group of atoms




covalently bonded together into a single unit
The unit obtains a charge
Most PA ions are negatively charged
 Oxyanions (anions): P, S, C, or N
covalently bound to one or more oxygens
Never occur independently, always
associated with ions of opposite charge
Only one PA is positively charged
 ammonium ion
Naming Ionic Compounds Containing a
Polyatomic Ion
 Must memorize name, formula and
charge (Table 5.6 on page 138). Look for
relationships between ions
 Most PA ions are oxyanions: The number
of oxygen atoms bonded to the same
element (e.g. P, S, or N) will determine
the name (suffix) of the ion
 ~ate is most common
 ~ite has one less oxygen bonded
Naming Ionic Compounds Containing a
Polyatomic Ion
 ~ate, ~ite pairs of ions
 The ion in the pair with the most oxygens is always the




~ate ion
The ion in the pair with one less oxygen is always
the ~ite ion
Ion pair with a 3- charge
 phosphate (PO43-), phosphite (PO33-)
Ion pair with a 2- charge
 sulfate (SO42-), sulfite (SO32-)
Ion pair with a 1- charge
 nitrate (NO3-), nitrite (NO2-)
Naming Ionic Compounds Containing a Polyatomic Ion
 Group 7A elements can form more than two




types of polyatomic ions (oxyanions)
Additional prefixes are used to differentiate
the ions
See page 138-9 (class text) and page 330
(lab text)
The number of oxygens attached to the
central atom has an effect on the name of
the ion
e.g. Polyatomic ions of chlorine, bromine
and iodine
Naming Ionic Compounds Containing a
Polyatomic Ion (Group VIIA)
 Example: Polyatomic ions of chlorine
1) -ate ion
 chlorate = ClO32) -chlorate ion with 1 more O than chlorate, use perprefix
 perchlorate = ClO43) - chlorate ion with 1 less O, use -ite suffix
 chlorite = ClO24) -chlorite ion with 1 less O, use hypo- prefix
 hypochlorite = ClO-
Naming Ionic Compounds Containing a
Polyatomic Ion
 Named the same way as binary ionic





compounds
Positive ion (metal) name is written first
Polyatomic ions name follows the metal
No prefixes are used in the name
Cation: Check to see if metal is single or
multiple cation
Use the name of the PA ion given in table
5.6 on page 138
Naming Ionic Compounds Containing a
Polyatomic Ion
• CaSO4
• calcium sulfate
• Ca2+ and SO42-
• Li2CO3
• lithium carbonate
• Li+, CO32-
• Al(NO3)3
• aluminum nitrate
• Al3+, NO3-
Writing Formulas for Compounds
Containing a Polyatomic Ion
 Formulas are written like binary ionic




compounds
Consider polyatomic ions as single units
with a certain charge
Obtain the correct ratio of cation to anion
to achieve a net charge of zero
Use parentheses if more than one of the
same PA unit is needed in a formula
Use subscripts to indicate the number of a
particular ion in a formula
Writing Formulas for Compounds
Containing a Polyatomic Ion
 Compound name is magnesium carbonate
 Mg2+ and CO32- are the ions
 Balance the charges
2
2
Mg
CO 3
1(2)  1(2)  0
 Write the formula
 MgCO3 is the formula using the
subscripts from the charge balance
Writing Formulas for Compounds
Containing a Polyatomic Ion
 Compound name is calcium nitrate
 Ca2+ and NO3- are the ions
 Balance the charges
2

Ca
NO 3
1 (2)  2(1)  0
 Write the formula
 Ca(NO3)2 is the formula using the
subscripts from the charge balance
Writing Formulas for Compounds
Containing a Polyatomic Ion
 Compound name is iron (III) sulfate
 Fe3+ and SO42- are the ions
 Balance the charges
3
2
Fe
SO4
2(3)  3(2)  0
 Write the formula
 Fe2(SO4)3 is the formula using the
subscripts from the charge balance
Writing Formulas for Compounds
Containing a Polyatomic Ion
 Compound name is ammonium phosphate
 NH4+ and PO43- are the ions
 Balance the charges

3
NH 4
PO 4
3 (1)  1 (3)  0
 Write the formula
 (NH4)3PO4 is the formula using the
subscripts from the charge balance
Naming Molecular Compounds
 Molecular binary compounds
 Composed of two nonmetal elements
 Naming a compound
 Use the full (element ) name for the first
nonmetal
 Add the –ide ending to the full name of the
second nonmetal
 Second nonmetal named like the nonmetal in
binary ionic compounds (anion)
 Indicate the number of atoms by adding
numerical prefixes
Prefixes Used in Naming Molecular (Covalent)
Compounds (page 140)
Subscript
1
2
3
4
5
6
7
8
9
10
Prefix used
mono~
di ~
tri ~
tetra ~
penta ~
hexa ~
hepta ~
octa ~
nona ~
deca ~
(Usually omitted on the first atom)
Naming Molecular Compounds
Prefix- Name of 1st nonmetal ___Prefix- Name of 2nd nonmetal + -ide
 In ionic compounds the subscripts are not mentioned in
the name
BaCl2
Na2SO4
barium chloride
sodium sulfate
barium dichloride
disodium sulfate
 Many compounds can exist for many pairs of nonmetallic
elements (e.g. nitrogen and oxygen)
NO
NO2
nitrogen monoxide
nitrogen dioxide
N2O
dinitrogen monoxide
Naming Molecular Compounds
 IF5
 iodine pentafluoride
 B2O3
 diboron trioxide
 NO3
 nitrogen trioxide
Naming Molecular Compounds
• AsCl3
• arsenic trichloride
• CO2
• carbon dioxide
• CO
• carbon monoxide
Naming Molecular (Covalent) Compounds
from their Chemical Formula
 When writing a formula from the name of
a binary molecular compound
 You must know (memorize) definition of
the numerical prefixes used in naming
covalent compounds (see page 140)
1) Write the symbols in order the
elements appear in the name
2) Identify the prefixes with the
appropriate subscripts
Examples
• Dinitrogen Pentoxide
• N2O5
• Tetraphosphorous Hexasulfide
• P4N6
• Iodine Heptasulfide
• IF7
• Monoiodine Heptasulfide
Naming Molecular Compounds
 A systematic name for a compound (IUPAC rules)
conveys information about the composition of the
compound
 Some compounds retain their “common” or “trivial”
names which are not based upon IUPAC rules
 These names were acquired before the systematic
nomenclature was established
Naming Acids
 Acids are hydrogen containing molecular
compounds that produce H+ and an anion when
dissolved in water
 They have a sour taste and dissolve some
metals
 They can be recognized by H written as the first
element in their formulas
Acids: HCl, H2S, H2SO4, HNO3
Nonacids: NH3, CH4, PH3, SiH4
Naming Acids
 Many molecular compounds containing hydrogen
dissolve in water to form H+ and an anion
 All acids produce H+ ion which gives acids their
characteristic properties
 The names of acids are derived from the names of
the anions produced by dissociation from the H+ ion
Naming Acids
 Binary acids have a H+ ion and a nonmetal
anion
 Oxyacids have a H+ ion and a polyatomic
anion (PA ion contains oxygen)
 There are three rules for naming acids
depending on whether the name of the
anion has the suffix -ide,-ate, -ite
Naming Binary Acids:
Anions Ending in -ide
 Use the prefix hydro- before the base (root)
name of the element
 Add the suffix -ic and the word acid to the
root name for the element
Hydro- + base name of nonmetal + -ic______ acid
 Example: HCl
 hydrochloric acid
 Example: HI
 hydroiodic acid
Naming Oxyacids
 An oxyacid is a molecular compound composed
of hydrogen, oxygen, and another nonmetal
 Exist as pure compounds and water solutions
 Produce H+ and a polyatomic ion when
dissolved in water HNO
H+ + NO 3
3
 For many nonmetals there is a series of
oxyacids and they differ only in the amount of
oxygen present
SO42-, SO32- , NO3-, NO2-
HClO4-, HClO3-, HClO2-, HClO-
Naming Oxyacids
Anions Ending in -ate
 A series of oxyanions
SO42-
SO32-
sulfate
sulfite
 Use the root name of the polyatomic ion
 If polyatomic ion ends in –ate, use –ic suffix
base name of polyatomic ion + -ic______ acid
 Example: H2SO4 (from SO42- ,sulfate ion)
 sulfuric acid (NO hydro- prefix used)
Naming Oxyacids
Anions Ending in -ite
 A series of oxyanions
SO42-
SO32-
sulfate
sulfite
 Use the root name of the polyatomic ion
 If polyatomic ion ends in –ite, use –ous suffix
base name of polyatomic ion + -ous______ acid
 Example: H2SO3 (from SO32- ,sulfite ion)
 sulfurous acid (NO hydro- prefix
used)
Summary of Writing Ionic
Compounds
 Summary of guidelines when writing binary ionic
compound
 The symbol of the cation always precedes the
anion
 The sum of the positive charges must equal the
sum of the negative charges:
A net charge of zero
 Whole numbers are written as subscripts to
indicate the number of each ion (or group) in the
formula
Formula Mass: The Mass of a Molecule or Formula Unit
 The formula mass is the sum of the atomic
masses of atoms present in a single formula unit
of a substance
 It is obtained by adding the atomic masses of
the constituent elements
 Each atomic mass is counted as many times as
the symbol occurs in the formula
Calculate the formula mass of Calcium Nitrate
Chemical formula: Ca(NO3)2
Formula Mass = 1 (formula mass of Ca) + 2 (formula mass of N) + 6 (formula mass of O)
= 40.08 amu + 2 (14.01 amu) + 6 (16.00 amu)
= 164.1 amu
 end