Writing formulas for Ionic Compounds
Review• Ionic compounds are composed of
cations (metals) and anions (nonmetals).
• Although they are composed of ions, ionic compounds are electrically neutral. The positive charge equals the negative.
• The positive charge and the negative charge attract one another. This is how the bond is formed.
Writing Binary Ionic Formulas
• The cation (Metal) with the correct oxidation number is written first
• The anion (nonmetal) with the correct oxidation number is written second
Binary Ionic Formulas
• Criss– cross the charges from superscripts to subscripts using only their absolute value
For Example
A+2 B-3
A3 B2
• Check to make sure the subscripts are in the lowest possible ratio to one another
For Example:
A4B6
A2B3
Example
Write the correct ionic formula for Barium and Chlorine.
Step 1: Write the cation (with correct oxidation number) first
And the anion (with the correct oxidation number second)
Barium and Chlorine
Ba+2Cl-1
Step 2: Criss-cross the superscripts to subscripts using only the absolute value
Ba+2Cl-1
BaCl2
Why is there no subscript for Ba?
Step 3: Make sure all subscripts are in the lowest possible ratio.
BaCl2
If there are in the lowest possible ratio you are done!!
Writing Formulas for Ternary Ionic Compounds
(3+ Elements)• Formulas of Ionic compounds Containing
Polyatomic Ions- it is imperative that you know the correct charges on these ions!!!!! When using multiple polyatomics, you must “keep it together” by putting it in parenthesis.
• Example: Ca3(PO4)2 - calcium phosphate
• The charge must still equal zero!
Write the correct formula for the following binary ionic compounds:
• Potassium and Iodine
• Calcium and Fluorine
• Aluminum and Sulfur• Iron(III) and Oxygen (watch out for the transition metal)
Writing Formulas for Ternary Ionic Compounds
(3+ Elements)
• Write the formula for:
sodium phosphate:
ammonium sulfide:
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