Writing Chemical Formulas for Ionic Compounds

The Crisscross Method

Essential Question

How do I correctly write chemical formulas for ionic compounds?

What information do I need?

Binary Compounds– Charges (Oxidation Numbers)

Compounds with more than two elements– Charge of the Polyatomic ion

Process for Writing Binary Process for Writing Binary Chemical Formulas for Ionic Chemical Formulas for Ionic

CompoundsCompoundsThe element or polyatomic ion with the positive The element or polyatomic ion with the positive oxidation number (cation) is written first.oxidation number (cation) is written first.The element with the negative oxidation number The element with the negative oxidation number (anion) goes second.(anion) goes second.Crisscross the absolute values of the oxidation Crisscross the absolute values of the oxidation numbers.numbers.

Ca+2 Cl-1

21

CaCl2

Let’s Practice!Let’s Practice!

Try writing the formula for a compound madefrom aluminum and sulfur.

Al+3 S-2

2 3Al2S3

Try writing the formula for a compound madefrom titanium and oxygen.

Ti+4 O-2

2 4Ti2O4

This last one is not quite right!

Now What Did We Do Now What Did We Do Wrong!Wrong!

For ionic compounds, when subscripts For ionic compounds, when subscripts are not in their simplest form, divide are not in their simplest form, divide them through by their common them through by their common multiple to get the empirical formula.multiple to get the empirical formula.

Ti2O4 TiO2

Empirical formulas show the simplest ratioof the elements in a compound. Molecular formulas show the actual numberof atoms in a molecule. (Covalent compounds only)

Process for Writing Non-binary Process for Writing Non-binary Ionic FormulasIonic Formulas

Some of the most common non-Some of the most common non-binary compounds include a binary compounds include a polyatomic ion.polyatomic ion.

Polyatomic ions are ions that are Polyatomic ions are ions that are made up of more than one element.made up of more than one element.

Examples are:Examples are:SO4

-2 Sulfate

OH -1 Hydroxide

Writing Formulas with Writing Formulas with Polyatomic IonsPolyatomic Ions

The method for writing formulas that The method for writing formulas that include polyatomic ions is similar to include polyatomic ions is similar to the one for writing binary formulas. the one for writing binary formulas.

Let’s write the formula for a Let’s write the formula for a compound made of Cucompound made of Cu+2+2 and CO and CO33

-2-2..

CuCu+2+2 COCO33-2-2

2 2

Taking it to its simplest form CuCO3

Let’s Try Another!Let’s Try Another!

Write the formula for a compound of Write the formula for a compound of AlAl+3+3 and SO and SO44

-2-2..

AlAl+3+3 SOSO44-2-2

2 3

Al2SO43

The 3 and 4 together make this formula confusing. There must be something more!

The Answer is Parentheses!The Answer is Parentheses!

When you have more than one When you have more than one polyatomic ion in a formula, you polyatomic ion in a formula, you must place parentheses around it.must place parentheses around it.

Al2SO43 Al2(SO4)

3

This formula shows that the compound ismade up of 2 aluminum atoms and 3 sulfatepolyatomic ions.

Conclusion• Writing ionic chemical formulas is

easy!All you have to do is