How to Write Ionic
Chemical Formulas

Formation of Ionic
Compounds

 Occurs when a metal loses all of its valence electrons
to a non-metal
 Metal becomes positively-charged (cation)
 Non-metal becomes negatively-charged (anion)
 Opposite charges hold the compound together!
 Electrostatic attraction
 An ionic compound is also called a formula unit
 When ions are combined together to form IONIC
compounds, the overall charge of the compound that
results MUST BE ZERO (neutral)!
Oxidation #’s and Ionic Compounds

 Depending on how many electrons an atom gains or
loses determines its charge (oxidation number)!
 Metal atoms lose 1, 2, 3, or 4 electrons and become +
ions (cations)
 Non-metals gain 1, 2, 3, or 4 electrons and become –
ions (anions)
 Remember – charges can be predicted by looking at
the group numbers on the Periodic Table!
The First Type of Ionic
Compound

 Type I Binary Ionic Compounds
 Formed between a metal with a SINGLE possible
(fixed) charge and a non-metal
The 5 Steps for Writing an Ionic
Compound Formula

 Write the symbols of the two elements
 Positive ion (cation) is written first; the negative ion (anion) is written
second
 In other words, the METAL is written first followed by the non-metal
 Write the charges of each as superscripts
 Use the Periodic Table to predict charge!
 Drop the positive and negative signs
 Criss-cross the superscripts so they become subscripts
 Compound must be neutral!
 To double check, multiply the subscript of the cation by its charge and
add the subscript of the anion multiplied by its charge – must equal
zero!
 Note – subscripts represent the number of atoms in one of the
compound
 Reduce to lowest subscripts when possible
Practice!

 Write a correct formula for the compound which
would form between:




Lithium and fluorine
Calcium and sulfur
Cesium and oxygen
Aluminum and oxygen
 Then, draw a picture of each compound. Some
things to think about:
 What do the subscripts represent?
 Are cations or anions bigger? Show this!
Type II Binary Ionic
Compounds

 Formed between a metal with MULTIPLE possible
charges and a non-metal
 Most transition metals (found in Groups 3-12) and
Group 14 metals form 2 or more ions
 Zn2+, Ag+, and Cd2+ form only one ion
Type II Binary Ionic
Compounds

Roman Numerals and Type II
Binary Ionic Compounds

 The Roman Numeral following the name of the
transition metal gives the charge of the metal cation
formed
 Examples
 Tin (IV) = Sn4+
 Mercury (II) = Hg2+
 Lead (II) = Pb2+
 Once charge of metal is determined, follow same
steps as Binary I Ionic Compounds and criss-cross
charges between metal and non-metal!
Practice!

 Write a correct formula for the compound which
would form between:




Iron (III) and oxygen
Tin (IV) and chlorine
Lead (IV) and oxygen
Tin (II) and sulfur
 Then, draw a picture of each formula unit. Some
things to think about:
 What do the subscripts represent?
 Are cations or anions bigger? Show this!
Ternary Ionic
Compounds

 Formed between a metal and a polyatomic anion
OR
 Formed between ammonium (NH4+) and a nonmetal
OR
 Formed between ammonium and a polyatomic anion
What is a Polyatomic
Ion?

More on Polyatomic Ions

 All polyatomic ions are found on your pink sheet!
 Note:
 All ions ending in –ate or –ite are polyatomic ions!
 All ions ending in –ide are elements from Periodic Table!
 Exception is cyanide (CN-) – a polyatomic ion
 Treat polyatomic ions as you would any ion
 Criss-cross to determine the ionic formula
 The only difference is that when you have more than one
of a specific polyatomic ion in a formula, you must encase
the ENTIRE POLYATOMIC ION in parentheses
 Example: Calcium hydroxide
Ca(OH)2
Practice!

 Write a correct formula for the compound which
would form between:




Calcium and nitrate
Magnesium and phosphate
Barium and hydroxide
Sodium and sulfate
 Then, draw a picture of each compound. Some
things to think about:
 What do the subscripts represent?
 Are cations or anions bigger? Show this!
Practice!

Steps to Name Type I
Binary Ionic Compounds


Remember, Type I binary ionic compounds are
formed between a metal with a SINGLE
possible (fixed) charge and a non-metal
 So to name a Type I ionic chemical formula:
1. Write the name of the metal
2. Write the root name of the non-metal and add the suffix “ide”
KCl → Potassium Chloride
 Note – number of metal atoms and non-metal atoms DOES
NOT matter when naming them!
Practice!

 MgBr2
 NaF
 ZnS
 K3N
Steps to Name Type II
Binary Ionic Compounds

 Remember, Type II binary ionic compounds are
formed between a metal with MULTIPLE possible
charges and a non-metal
 To name a Type II ionic chemical formula:
1. Write the name of the metal
2. Write the appropriate Roman Number in
parentheses to indicate the charge of the metal
3. Write the root name of the non-metal and add the
suffix “-ide”
How Do You Know What
Roman Numeral to Use?

 To tell what the charge of the metal is, LOOK AT
THE NON-METAL!
 Remember, the overall charge of an ionic compound
is ZERO
 Therefore, the metal has to EQUAL the charge of the
non-metal
CuCl2
So, Cu has a
charge of +2!
Each Cl has a -1
charge, and there
are 2 of them
Practice!

 FeCl2
 Cu2S
 Hg2I2
 PbO2
What’s a Tertiary Ionic
Compound Again?

 Remember, tertiary (ternary) ionic compounds are
formed:
 Between a metal and a polyatomic anion
OR
 Between ammonium (NH4+) and a non-metal
OR
 Between ammonium and a polyatomic anion
Steps to Name Tertiary
Ionic Compounds

1. Write the name of the metal OR ammonium (if
present)
2. Now, look at the anion
 If anion is a monatomic non-metal (ex. Cl), write the
root name of the non-metal and add the suffix “-ide”
 If anion is polyatomic, simply name the polyatomic
ion using your pink sheet!
Practice!

 Al2(C2O4)3
 Na2CO3
 FeSO4
 Ag2SO3
 NH4Cl