Naming Compounds and Writing
Formulas for Ionic Compounds
Chapter 9 Section 2
Naming Binary Ionic Compounds
• Composed of two elements
• Cation name followed by anion name
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NaF
CaCl2
Li3N
Sodium fluoride
Calcium chloride
Lithium nitride
Naming Binary ionic compounds with
transition metals
• Recall transition metal ions have multiple charges
• First start with ion that only has one possible charge
▫ “position of strength”
• Then determine transition metal ionic charge.
• Example: CuO
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Copper and oxygen
Copper can be +1 or +2…
…but Oxygen is always -2. Position of strength!
1:1 ratio means this copper must be +2 to cancel out
the -2 of the oxygen atom.
▫ Copper (II) oxide
You try it!
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SnS2
FeCl3
SnF2
CuI2
CoI2
CuI
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Tin (IV) sulfide
Iron (III) chloride
Tin (II) fluoride
Copper (II) iodide
Cobalt (II) iodide
Copper (I) iodide
Writing Formulas for Binary Ionic
Compounds
• Write the symbol for the cation followed by the
symbol for the anion.
• Add whatever subscripts are needed to balance
the charges.
• Potassium chloride
• Calcium bromide
A little trickier…
• Iron (III) oxide
▫ Iron (III) is Fe3+
▫ Oxide is O2-
• How do you balance this?
• Crisscross method, or finding the least common
multiple method.
• Iron (III) Oxide (Rust)
▫ LCM Method
▫ 3x2 = 6 (least common multiple)
▫ Two Fe3+ cations have a net charge of:
 +6
▫ Three O2- anions have a net charge of:
 -6
▫ Balanced formula for Iron(III) Oxide is Fe2 O3
Crisscross Method
• Numerical charge of each ion is crossed over and
used as a subscript for the other ion
• The signs of the numbers are dropped.
• Fe3+
O2-
• Fe2O3
• 2(3+) + 3(2-) = 0
Crisscross doesn’t always work…
• Calcium Sulfide
• Ca2+
S2-
• Ca2 S2
• Remember: formulas for ionic compounds
show the lowest whole-number ratios of ions
• The correct formula for Calcium sulfide is CaS
• 1(2+) + 1(2-) = 0
• Crisscross method unnecessary in this case
Ternary Ionic Compounds
• Ternary Compound: A compound that contains
atoms of 3 different elements
▫ Most ternary ionic compounds contain a
polyatomic ion
▫ THIS IS WHY YOU NEED TO HAVE THEM
MEMORIZED!!! You need to recognize
polyatomic ions quickly
Example: write the formula from the
name
• How would you write the formula for calcium
nitrate?
• Write the formula (symbol and charge) for each ion
• Balance the charges (hint: use crisscross here)
• Ca2+ and NO3• Ca(NO3 )2
• Include parenthesis around the polyatomic ion
followed by the subscript of how many of those ions
are needed to balance the charge of the compound
More examples
▫ If you only need one of the polyatomic ions to
balance the charge of the compound, you do not
need to use parentheses
▫ Calcium Carbonate
 Ca2+ and CO32 CaCO3
 No parentheses
▫ Strontium Sulfate
 Sr2+ and SO42 SrSO4
▫ Lithium Carbonate
 Li+ and CO32 Li2 CO3
Write the formulas for the following
• Potassium Sulfate
▫ K+ and SO42▫ K2 SO4
• Magnesium Hydroxide
▫ Mg2+ and OH▫ Mg(OH)2
Naming Ternary Ionic Compounds
• Name the cation first, followed by the anion
• Determine the charge of the polyatomic ion.
• Balance the polyatomic anion by adding an
appropriate subscript for the cation.
• K2Cr2O7
• Composed of K+ and Cr2O7-2
• potassium dichromate
Name the following Compounds
• LiCN
▫ Lithium Cyanide
• Sr(H2PO4 )2
▫ Strontium dihydrogen phosphate
• (NH4 )2 C2O4
▫ Ammonium Oxalate
• Fe(ClO3)3
▫ Iron (III) Chlorate