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!

Practice

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: