IONIC COMPOUNDS
Formulas & Nomenclature
Formulas for Ionic Compounds
• Formula Unit – the simplest ratio of the ions
represented in an ionic compound
Because the total number of electrons
gained by the nonmetallic atoms must equal
the total number of electrons lost by the
metallic atoms, the overall charge of the
formula unit is zero.
Determining Charge
• Binary IONIC compounds are composed of
positively charged monatomic ions of a metal
and negatively charged monatomic ions of a
non-metal.
Monatomic ion – a one atom ion, such as
Mg+2 or Br-1
Oxidation number – the charge of a
monatomic ion
Practice Problems
A.
B.
C.
D.
E.
F.
List the oxidation numbers for each of the
following ions.
Lithium ion
G. Oxide ion
Phosphide ion
H. Calcium ion
Bromide ion
I. Beryllium ion
Potassium ion
J. Francium ion
Cesium ion
K. Nitride ion
Iodide ion
L. Selenide ion
• In the chemical formula for any ionic
compound, the symbol of the cation is always
written first, followed by the symbol of the
anion. Subscripts, which are small numbers
to the lower right of the symbol, are used to
represent the number of ions of each element
in an ionic compound. If no subscript is
written, it is assumed to be one.
Example Problem 1:
• The ionic compound formed from potassium
and oxygen is used as a dehydrating agent
because it reacts readily with water.
Determine the correct formula for the ionic
compound formed from potassium and
oxygen.
Potassium + Oxygen
• Step 1: Analyze the problem
The first thing to do is write the symbol and
oxidation number for each ion involved in the
ionic compound
+1
K
-2
O
“cross charges”
Potassium + Oxygen
• Step 2: Solve for the unknown
A potassium ion loses one electron while an oxygen
atom gains two electrons. If combined in a oneto-one ratio, the number of electrons lost by
potassium will not balance the number of
electrons gained by oxygen. To have the same
number of electrons lost and gained, you must
have two potassium ions for every oxide ion.
The correct formula is:
K 2O
Potassium + Oxygen
Step 3: Evaluate the answer
Check to be sure the overall charge on the
compound is ZERO
K2O
+1
K
-2
O
2(+1) + 1(-2) = 0
Practice Problems
• Write the correct formula for the ionic
compound composed of the following pairs of
ions:
1. Potassium & iodide
2. Magnesium & chloride
3. Aluminum & bromide
4. Cesium & nitride
5. Barium & sulfide
Practice Problems
• Write the correct formula for the ionic
compound composed of the following pairs of
ions:
1. Potassium & iodide
KI
2. Magnesium & chloride
MgCl2
3. Aluminum & bromide
AlBr3
4. Cesium & nitride
Cs3N
5. Barium & sulfide
BaS
Oxidation Numbers for Selected
Transition Metals
• Silver
• Zinc
• Cadmium
+1
+2
+2
• All other transition metals have more than
one oxidation number and have to be treated
differently
Naming Binary Ionic Compounds
• A binary ionic compound is made up of a
+ metal (cation) and a – nonmetal (anion)
The ending for the nonmetallic ion is –ide.
Steps for Naming Binary Ionic
Compounds
• 1. Name the metal (cation) first and the
nonmetal (anion) second. Remember, the cation
is ALWAYS written first in the formula.
• 2. Monatomic cations use the metal name.
• 3. Monatomic anions take their name from the
root of the element name plus the suffix
-ide.
• Name of Metal
(+ ion/cation)
+ Root of nonmetal
+ “-ide” ending
(- ion/anion)
**no prefixes in ionic compound names**
Example:
Write the name for BeCl2
• Step 1: Analyze the problem:
Find the names of the elements associated with
each symbol.
Be – beryllium
Cl - chlorine
• Step 2: Solve for the unknown
Name the metal. Name the root of the
nonmetal and add the “-ide” ending.
Beryllium chloride
• Step 3: Evaluate the answer
Make sure the name makes sense and is correct
for the formula.
Practice Problems
Write the correct name for the following ionic
compounds:
• NaI
• ZnO
• AlBr3
Practice Problems
Write the correct name for the following ionic
compounds:
• NaI
• ZnO
• AlBr3
sodium iodide
zinc oxide
aluminum bromide