Writing Ionic Formulas
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Writing Ionic Formulas
Ionic Compounds Things you should know:
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 IonsPredicting Charges on IonsKNOW THESE !!!!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. 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. Ex: potassium oxide = K2O
Your Turn!
What is the formula of barium iodide? 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? 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. 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. Poly = “many” Atomic = refers to
atoms
Polyatomic Ions-continued= a group of atoms covalently
bonded together that has a net charge.
Examples:NH4
+ Ammonium C2H3O2
- AcetateCN- CyanideO2
2- PeroxideNO3
- Nitrate NO2
- NitriteSO4
-2 SulfateSO3
-2 Sulfite
Together, the group has a charge. Like a molecule, the O and H atoms stay bonded together and act as one particle.
1–
HO
OClO
1–
One chlorine atom and two oxygen atomscovalently bonded, together carrying a 1- charge
ClO2-
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 TogetherTransitional 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
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