22.3 – Writing formulas and
Naming Compounds
Objectives
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Explain how to determine oxidation
numbers
Write formulas and names for ionic
compounds
Binary Ionic Compounds
• A compound composed of 2 elements
• To write a chemical formula for a
binary compound, you need to know:
• which elements are involved
• how many electrons they lose or gain to
become stable
Oxidation Numbers Transition Elements
• Because they can have
multiple oxidation numbers, we
have to express that number in
the name with a Roman
Numeral
• For example, the oxidation
number of iron in
iron (III) oxide is 3+.
Write Formulas for these
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Potassium ion
Magnesium ion
Copper (II) ion
Chromium (VI) ion
Barium ion
Mercury (II) ion
Ionic Compounds are
NEUTRAL
• Although the individual ions in a
compound carry charges, the
compound itself is neutral
• A formula must have the right number
of positive ions and the right number of
negative ions so the charges balance
Writing Formulas

Write the formula for calcium chloride.
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Calcium is Ca+2
Chloride is Cl-1
Ca+2 Cl-1 would have a +1 charge.
Need another Cl-1
Ca+2 Cl 1- Cl 1CaCl2
Step By Step – Writing Formulas
1. Write the symbol of the element (or
polyatomic ion) with the positive oxidation
number or charge
2. Write the symbol of the element (or
polyatomic ion) with the negative oxidation
number or charge
3. The charge (without the sign) of one ion
becomes the subscript of the other ion.
•
Reduce the subscripts to the smallest whole
numbers that retain the ratio of ions.
Criss-Cross Method
Polyatomic Ions
• Groups of atoms that stay together and
have a charge
• A positively or negatively charged,
covalently bonded group of atoms
• Polyatomic ions as a whole contains two
or more elements
Polyatomic Formulas
When writing out the formula for
polyatomics, treat the polyatomic as a
whole and place () around the symbol
 Then place any subscripts outside of
the () because the subscript applies to
the entire polyatomic
YOU NEVER CHANGE THE FORMULA
OF THE POLYATOMIC

Name
ammonium
acetate
chlorate
chlorite
cyanide
hydroxide
nitrate
carbonate
sulfate
phosphate
Symbol
NH4+1
C2H3O2-1
ClO3-1
ClO2-1
CN-1
OH-1
NO3-1
CO3-2
SO4-2
PO4-3
Write the formulas for these
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Lithium sulfide
Tin (II) oxide
Tin (IV) oxide
Magnesium fluoride
Copper (II) sulfate
Iron (III) phosphide
Gallium nitrate
Iron (III) sulfide
Barium nitrate
In-Class
Assignment/Homework
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22.3a Criss Cross WS
22.3a Criss-Cross practice