Writing Ionic Formulas
Ionic Compounds
 Things you should know:
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Ionic = metal-nonmetal combo of elements
Metal: loses e- to become stable; positive
ions
Nonmetal: gains e- to become stable;
negative ions
The charge value (1, 2, or 3) depends on
how many e- were lost or gained.
The # lost or gained was the # needed to
be “full”.
Predicting Charges on Ions
KNOW THESE !!!!
+1 +2
+3
-3 -2 -1
0
Method One: electron dot
diagrams
The # of dots shown = # of
valence e-.
Column # = # of valence e-.
e- are gained or lost in the #
needed for each atom/ion to
be stable.
e- dot diagrams for some
common elements
Use e- dot diagrams to show the
transfer of e- to make the metal and
nonmetal atoms stable.
e- dot diagram for Potassium (K)
e- dot diagram for oxygen (O)
Transfer of e- to form potassium oxide
Writing the Formula
 If the ratio is 1:1, no subscript is needed.
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Ex: sodium chloride = NaCl
 If the ratio is NOT 1:1 use a subscript for
each element which has more than 1 ion
involved in the transfer.
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Ex: potassium oxide = K2O
Your Turn!
 What is the formula of barium
iodide?
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Show the e- dot diagram of each element.
Show the transfer of e-.
Write the formula based on the ratio of the
ions.
Barium Iodide
e- dot diagram for Barium (Ba)
e- dot diagram for iodine (I)
Transfer of e- to form barium iodide
Formula
BaI2
Method 2: using charges of the
ions
Ion charge for Aluminum (Al)
Ion charge for chlorine (Cl)
“Adding” the ions to get a neutral
compound
The formula
AlCI3
Your Turn!
What is the formula of lithium
sulfide?
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Determine the ion charge of each element.
Determine the # of each ion needed to
form a neutral compound.
Write the formula based on the ratio of the
ions.
The Formula
Li2S
Transition Metals
 Transition metals = B column elements
 Charge can vary. It may be: +1, +2, +3,
or +4.
 A number in parentheses following the
name of the metal gives the ion charge.
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Ex: Iron (II) = Fe+2
Iron (III) = Fe+3
Teacher Example
 What is the formula of chromium (III)
sulfide?
Ion charge for Chromium (Cr)
Ion charge for sulfide
“Adding” the ions to get a neutral
compound
The formula
Cr2S3
Your Turn!
 What is the formula for silver (I)
chloride?
The formula
AgCI
Polyatomic Ions
 Not all ionic compounds are composed
of metal and nonmetal elements.
 What?!?!
 All ionic compounds ARE composed of
positive and negative ions.
 Some ions are composed of 2 or more
elements.
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Poly = “many”
Atomic = refers to atoms
Polyatomic Ions-continued
 = a group of atoms covalently
bonded together that has a net
charge.
 Examples:
NH4+
Ammonium
C 2 H 3 O2 Acetate
CNCyanide
O22Peroxide
NO3Nitrate
NO2Nitrite
SO4-2
Sulfate
SO3-2
Sulfite
1–
O
H
Together, the group has a charge. Like a
molecule, the O and H atoms stay bonded
together and act as one particle.
ClO21–
O
Cl
O
One chlorine atom and two oxygen atoms
covalently bonded, together carrying a 1- charge
Writing a formula that contains
a polyatomic ion
 Use the charge method. “Adding” the
charges of the ions, the compound must
be neutral.
 Compound: Magnesium hydroxide
Magnesium hydroxide
Ion charge for Magnesium (Mg)
Ion charge for hydroxide (OH-)
“Adding” the ions to get a neutral
compound
The formula
Mg(OH)2
Your Turn!
Write the formula for lithium
phosphate.
The formula
LiPO4
Let’s Put It All Together
Transitional metal and
Polyatomic Ion
 What is the formula of chromium (III)
sulfate?
Ion charge for Chromium (Cr)
Ion charge for sulfate
“Adding” the ions to get a neutral
compound