Chapter 19: Molecules
and Compounds
Section 19.2
Chemical Formulas
19.2 Chemical Formulas
All
compounds
have an
electrical
charge of
_____; that is,
they are
______.
To be stable…
the ____
electrical
_____ of
compounds
must be
______.
Oxidation Numbers
An oxidation number indicates how
many electrons are ______ or ______
during the bonding process.
Mg loses 2 electrons so it
has an oxidation number
of +2
Oxygen gains 2 electrons
so it has an oxidation
number of –2.
Common Oxidation Numbers
1+
0
2+
3+ 3- 2- 1-
Transition Metals
Copy on your Periodic Table
______
______ have
more than
one
oxidation #.
_____ _____
show
oxidation #.
Writing Chemical Formulas
– monatomic ions
1. Symbol
of (+) ion always
written ____.
2. Symbol of (–) ion always
written ____.
3. Add _______ so sum of
oxidation #’s is zero.
Example:
Write formula for
______ (2 element)
compound made of
iron(III) and oxygen.
1. Find oxidation #’s of
elements:
iron(III) =
oxygen =
3+
Fe
2O
How do you make a
cmpd electrically neutral?
3+
Fe ions
Calculate
needed
2to combine with O ions
to make electrical
charges equal zero.
2
3+
(Fe )
added to 3
2(O )
=0
2 (3+) added to 3 (2-) = 0
2. To determine ratios, write
chemical formulas…Use
the ____________
_______
3+
Fe
O
2
3
2-
Writing Chemical Formulas
with polyatomic ions
“_______” means many.
See page 337: Oxidation
#’s for polyatomic ions.
Each polyatomic ion is
treated like a ______ ion.
Common Polyatomic Ions
Using Polyatomics
How would you write the chemical formula for
Calcium with Hydroxide.
 Ca is located in Group 2 so it
has a +2 oxidation number
 OH is a polyatomic ion with a charge of –1
 In order to make the total charge 0 you
need
(Ca+2) x (1) = +2
+ (OH-) x (2) = -2
0
Example: Write formula
for aluminum sulfate.
1st ion is always (+). Use
PT to find oxidation #.
Aluminum = Al3+
nd
2
ion is always (-). Use
Table 19.2.
Sulfate = SO42-
LCM of 2 and 3? 6
How many of each ion
are needed?
3+
(Al ) x 2 = 6+
0
2(SO4 ) x 3 = 6-
Write chemical formula
Al2(SO4)3
Don’t change subscripts in
polyatomic ion!! Use ( )
________ method
Al3+
2
SO423
Naming binary
ionic compounds
st
1
Write name of
element
or polyatomic ion.
Write _____ name of 2nd
element and add -ide.
Exs: chlor-ine = chlor-ide
phosph-orus = phosph-ide
Naming Formulas
Write the name for the first element in
the compound.
Write the root name of the second
element.
Add the ending –ide to the root name
Naming BaF2
 First element is Barium
 Second element is Fluorine (root is
fluor)

The name is Barium Fluoride
Naming Polyatomic Ions
For polyatomic ions: (use chart)
 Write the name for the first positive
ion.
 Write the name for the negative ion
Naming MgCO3
 Positive ion is Magnesium
 Negative ion is Carbonate (see chart)
 The name is Magnesium Carbonate
Naming binary
_______ compounds
Add -ide to the neg. ion
(anion)
Add prefix (pg 340)
st
If only one atom of 1
element, don’t use mono-
Examples:
CO
carbon monoxide
CO2
carbon dioxide
PCl5
phosphorus pentachloride
N2S6
dinitrogen hexasulfide
Page 340 in Text
Examples
SiF4
Silicon
tetrafluoride
N2S3
Dinitrogen
trisulfide
P2O5
Diphosphorus
pentoxide
SF6
Sulfur
hexafluoride
Empirical vs
Molecular formulas
_________ formula –
actual # of atoms of each
element in a compound.
_________ formula –
simplest whole number
ratio of elements in cmpd.
Example:
Molecular formula -sugar
C6H12O6
Empirical formula -sugar
CH2O