Chapter 3
Molecules and Compounds
Molecules and Compounds Chemical Formulas
1:1
CO
1:2
H2O
1:3
NH3
2:3
1:4
Al2O3 CH4
etc
C + 4 H = CH4
Molecular model
Molecular Models
Ethanol
Molecular Models
Some compounds are IONIC - electrons are
TRANSFERRED from a metal to a nonmetal; the
compound is held together ELECTROSTATICALLY
Coulomb’s Law: force = k (n+)(n-)/d2
Some compounds are COVALENT - electrons
are SHARED between two atoms
e
+
+
e
E. g. carbon dioxide
This commonly occurs for two or more NONMETALS
The Covalent Bond
Ions
• An atom or group of atoms with a net
charge caused by the net loss or gain of
electrons
• CATION = positively charged ion
• ANION = negatively charged ion
Formation of Cations & Anions
Predicting Whether an Atom Will Form a Cation or
Anion in Order to Make an Ionic Compound
Metals LOSE electrons to form CATIONS, nonmetals
GAIN electrons to form ANIONS
Valence Electrons in Ionic Compounds
• The A-group (representative) elements
follow the OCTET RULE; they obtain an
inert gas valence (outer) shell that contains
8 electrons
• Metals - lose # electrons = group number
e.g. Ca  Ca2+ + 2e- (Ar outer shell)
• Nonmetals - gain electrons = 8 - group #
e.g. N + 3e  N3- (Ne outer shell)
Valence Electrons for Covalent Compounds
• Covalent compounds form between
two or more nonmetals
• In this case the nonmetals can either
LOSE all of their valence electrons,
or • GAIN enough electrons to obtain an
OCTET
Examples SO3 - oxygen (VIA) gains 8-6 = 2
O2- ion forms
- sulfur (VIA) loses all 6
S6+ ion forms
NOTE: There is a rule that states that oxygen is
ALWAYS -2. These rules are coming up!
Example 3.3 - Predicting Ion Charges
When Forming Ionic Compounds
• Metals lose electrons, Nonmetals gain them
• Al = group IIIA metal, so LOSES 3
ELECTRONS
Al  Al3+ + 3e• S = group VIA nonmetal, so GAINS 8 - 6 =
2 ELECTRONS
S + 2e  S2-
Polyatomic Ions
• Contains 2 or more atoms COVALENTLY
bonded, and the complete unit contains a net
charge, e.g. nitrate, NO 3
Polyatomic Ion Examples
2
2
3
NO2 , CO3 , SO4 , PO4
NO2- ion
CO32- ion
Ionic Compound = Metal + Nonmetal
or a Metal + Polyatomic Ion
• Compound held together electrostatically
• Very strong forces hold the lattice together, so
ionic cmpd’s have very high melting points
NaCl crystal lattice
m.p. = 800 oC
Predicting Formulas of Ionic Compounds
Balance positive and negative charges to produce a
neutral molecule
Ca2+
+
Cl

Ca2+ + CO32- 
Ca2+ + PO43- 
Al3+
+
2
O

Oxidation Numbers
• A number assigned to each element in
a compound in order to keep track of
the electrons during a reaction
Mg2+ = +2
Cl- = -1
O2- = -2
N3- = -3
Rules for Assigning Oxidation Numbers
(Chap. 5, p. 207)
Rules higher up take precedence over lower rules
1 The O.N. for an atom in its pure, uncombined
state = 0.
2 The sum of the O.N.’s for a neutral molecule =
0. For a polyatomic ion, the sum = charge.
Rules cont’d
3 Group IA = +1
Group IIA = +2
4 H = +1 UNLESS combined with IA or IIA,
then = -1
5 Oxygen = -2
6 For binary ionic compounds only Group VA = -3
Group VIA = -2
Group VIIA = -1
Examples
• P4
• Al2O3
•
•
•
•
MnO4
NaH
Na2SO3
Mg3N2
Chemical Nomenclature Examples
(More Detail in Lab)
• Ionic Compounds
NaCl
Al2S3
FeSO4
KClO3
sodium chloride
aluminum sulfide
iron(II) sulfate
potassium chlorate
• Covalent Compounds
SO2
P2O5
N2O
sulfur dioxide
diphosphorus pentaoxide
dinitrogen oxide
The Mole - The mole is the chemist’s
counting unit
pair = 2
Dozen = 12
Gross = 144
Ream = 500
Avogadro’s Number (NA) = 6.022 X 1023
Where Does Avogadro’s Number Come From?
• By definition, 12C = 12.000 amu
• How many particles does it take to have
12.000 grams of 12C ?
• NA = 6.022 X 1023 (as determined by
experiment)
Significance of the Mole
Mass in
amu’s
Mass in
grams/mole
NA of carbon atoms weighs
NA of iron atoms weighs
Molar Mass - the mass in grams of one
mole of any element
• Molar mass of sodium (Na)
= mass of 1 mol of Na atoms
= 22.99 g/mol
= mass of 6.022 X 1023 Na atoms
• Molar mass of lead (Pb)
= mass of 1 mol of Pb atoms
= 207.2 g/mol
= mass of 6.022 X 1023 Pb atoms
Mass
Moles Conversion
Moles to Mass
Mass to Moles
moles • grams = grams
1 mole
grams • 1 mole = moles
grams
Molar mass
1 / Molar mass
Example 3.6 - Mass to Moles
How many moles are represented by 125 g of
silicon, an element used in semiconductors?
Example 3.7 - Moles to Mass
What mass, in grams, is equivalent to 2.50
mol of lead (Pb) ?
Mole Calculation Using Density
The graduated cylinder in the photograph contains
25.0 cm3 of Hg. If the density of Hg = 13.534 g/cm3
at 25 oC, how many moles of Hg are in the cylinder?
How many atoms of Hg are there?
Molar Mass of a Compound
Sum up the molar masses of each
atom in the compound
HC2H3O2
Example 3.9 - Molar Mass & Moles
You have 16.5 g of the common compound oxalic
acid, H2C2O4. Calculate 1. The number of moles
2. The number of molecules
3. The number of C atoms
4. The mass of one molecule
Other Fun Stuff
46.07 g contains 2(12.01) = 24.02 g of carbon
1(16.00) = 16.00 g of oxygen
6(1.008) = 6.05 g of hydrogen
C2H5OH
MW = 46.07
1 molecule contains 2 carbon atoms
1 oxygen atom
6 hydrogen atoms
1 mole contains 2 moles of carbon atoms
1 mole of oxygen atoms
6 moles hydrogen atoms
Conversion factors for C2H5OH • 2(12.01) g C/ 46.07 g C2H5OH
OR
24.02 g C/ 46.07 g C2H5OH
• 6 moles H/ mole C2H5OH
• 1 mole oxygen/ 2 moles C
More Problems How many grams of Na are there in 200. g of
Na2CO3 ?
How many moles of oxygen are there in 25.0 mol
of SO2 ?
More Problems How many aluminum atoms are there in 150. g of
Al2O3 ?
How many oxygen atoms are there in 500. mL of a
30.0 % solution of H2SO4 with a density of 1.250
g/cm3 ? (MW = 98.1)
Percent Composition from a Known Formula
NH3 MW = 17.03 g/mol
%N=
%H=
Empirical & Molecular Formulas
Empirical = simplest ratio
of atoms in the molecules
Molecular = actual ratio
Calculating Empirical Formulas
Formulas of unknown compounds are determined
from the percent composition of each element by
mass.
Assume 100 g and divide
by atomic weight
Divide by fewest
number of moles
Calculating Molecular Formulas
The molecular weight must be known. It is obtained
from a separate experiment
Benzene empirical formula = CH
formula weight = 12.01 + 1.008
= 13.018
If the MW = 78.11, then what is
the molecular formula?
Example 3.10
Eugenol is the active component of oil of cloves. It has a
MW of 164.2 g/mol and is 73.14 %C and 7.37 %H; the
remainder is oxygen. What are the empirical and molecular
formulas?
Another example Vanillin is a common flavoring agent. It has a molar mass of
152 g/mol and is 63.15 %C and 5.30 %H; the rest is oxygen.
What are the empirical and molecular formulas?