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: