Chapter 3
Molecules &
Compounds
3.1 Molecules, Compounds
and Formulas
 Empirical
Formulas
Definition: the simplest whole number
ratio for the formula of the elements in
a compound
 Examples: NaCl, MgCO3, AlBr3
 Ionic Compounds: a compound
consisting of oppositely charged ions
held together by electrostatic
attractions

Molecular Formulas
 Definition:
a formula consisting of the
exact number of atoms of each element
in a molecule of a compound
 Examples: C6H12O6, CO2, SiO2
 Condensed structural formulas – formulas
depicting some of the bonding by
groupings CH3OH , CH3CH2CH2Cl
Molecular formulas
 Condensed
structural formulas – formulas
depicting some of the bonding by
groupings CH3OH , CH3CH2CH2Cl
Structural formulas
A
two dimensional representation of
the atoms and bonds
 http://www.shef.ac.uk/~chem/vepr/
3.3&3.4 Ionic Compounds: formulas,
Names and properties
 Definition:
compounds containing a
cation and an anion electrostatically
bonded (due to transfer of electrons)
 Binary Ionic Formulas – binary means two
ion types in the compound
Charges for Main Group Elements
+1,+2,+3, skip, -3,-2,-1, NG

Examples
 Li
and O
 Now
you try…
Binary compound naming
The metal goes first always – element name
The nonmetal goes second with an –ide at
the end 
MgS – magnesium sulfide
Keep going 
6. Transition Metals
 Stock
system – a system of using Roman
Numerals to name transition metal
cations; the Roman Numeral is the charge
on the metal ion
Ex 3.3
Ag (I) and I
Ni (II) and Cl
One more time…keep on going
Polyatomic ions:
 Multi-atom
ions that are covalently
bonded within the ion
 Memorize the flash cards provided
Ex 3.4
 NaNO3
 NaNO2
 Your
turn …
Properties of ionic compounds
 Characterized
by the electrostatic
attraction between elements
 Solids at room temperature
 High melting points
 Brittle
 Conduct electricity in the molten state
 Conduct electricity as solutions
(electrolytes)
Molecular Compounds:
formulas, names and
properties
 Properties:
smaller compounds are gases
and liquids at room temperature. Large
compounds have low melting points, are
brittle, and characterized by covalent
bonding
Writing formulas: prefixes
 Mono
 Di
 Tri
 Tetra
 Penta
1
2
3
4
5
 Hexa
 Hepta
 Octa
 Nona
 Deca
6
7
8
9
10
Name the following molecular
compounds
 P2O7
 SCl3
 CF4
 N2O5
 CO
 CO2
 H2O
Write the formulas for:
 Diboron
trioxide
 Tetraphosphorus trisulfide
 Arsenic pentoxide
 Phosphorus trichloride
 Diarsenic heptoxide
3.6 Formulas, Compounds,
and the MOLE
1.
How many moles of CaCO3 can produced
starting with 4.22 x 1025 atoms of oxygen and
an excess of carbon and calcium atoms?
2. How many atoms of each kind are
present in 3.44 moles of dinitrogen
heptoxide? N2O7
Determining Compound
Formulas
 Percent
composition: The percentage of
the elements of a compound or parts of a
mixture; must total = 100%
 Determine
the percent by mass of each of the
elements in CO2
Determine the percent composition of
sodium bicarbonate.
Intensive property
 Property
that is independent of the
size of the sample of a substance
 Examples: density, solubility,
reactivity with acid, melting point,
boiling point

Meaning of subscripts
 The number of atoms present in the molecule
or ionic compound
 NOTE: empirical formula gives the smallest
whole number ratio
Meaning of subscripts
 Represent
the number of atoms or
ions present in the molecule or ionic
compound
 NOTE: empirical formula gives the
smallest whole number ratio
Empirical and molecular
formulas from experimental
data
Steps:
1. Mass percents are equivalent to masses
in grams
2. Convert masses to moles
3. Find the mole ratio (divide by smallest)
4. These ratios are the subscripts in the
empirical formula
Ex3.11Cassiterite is 78.8% tin and 21.2 %
oxygen Determine is empirical formula.
Ex3.12 Analysis of 47.25 mg sample of
aluminum chloride showed that it contained
9.56 mg of Aluminum. Determine the
empirical formula of the compound.
Ex 3.13 A molecular substance is 83.6% carbon
and the remaining portion is hydrogen only.
Determine its molecular formula if the formula
mass is about 86 g/mole.
Ex 3.14 A compound of silicon and
fluorine is found to contain 33.01%
SILICON AND 66.99% FLUORINE. Its
molar mass is 170 +/- 5 g/mole.
Determine the empirical and molecular
formulas.
Hydrated compounds
 Ionic
compounds that have water within
the crystalline structure

CuSO4.5H2O

Ex 3.15 A hydrate of magnesium chloride
consists of 36.2% water What is the correct
formula for this hydrate

Answer :MgCl2. 3 H2O
Ex 3.16 Determine the number of waters of
hydration for nickel (II) chloride from the
following lab data.
Initial mass of nickel(II)chloride hydrate
Final mass of anhydrous salt
0.235g
0.128g
 NiCl2
0.128 g (1 mol/129.5984g) = 9.8766 x10-4 mol
X
0.107 g (1 mol/18.016 g)

H2 O
= 0.005939 mol
Divide by the smallest to get ratio 6:1
 NiCl2 . X
H2 O
HOMEWORK
#1 11, 13, 33, 35, 37, 89 Worksheet
#2 41, 43, 45, 47, 49, 51, 53
#3 55, 57, 77, 83
#4 61,63, 85, 87, 93, 97, 99
#5 105, 107, 111, 113