Chemistry Formulas and Equations

Chemistry - How to Write Covalent and Ionic Chemical Formulas

Writing chemical formulas is a necessary skill if you are going to be successful in chemistry. Balancing equations, predicting reactions and calculating stoichiometric problems all rely on your ability to properly write a chemical formula. If you make a mistake in the formula it will affect the balancing and the mole ratios used to perform stoichiometric calculations. So take some time to learn how to properly write chemical formulas, you'll be glad you did.

What You Need to Know...

How to use the Periodic Table

How to use subscripts

How to determine ionic charges

Polyatomic ions

Metals with multiple charges

Greek and Latin numeric prefixes

The General Rule

Most compounds consist of two parts (binary) and are classified as ionic or covalent. The general rule for naming and writing compounds is to put the more metallic element first followed by the less metallic element. Remember to use subscripts to show the number of atoms or ions present in the formula.

Writing Ionic Formulas

When writing the chemical formula for ionic compounds place the cation first, then the anion and check to see that the charges balance to zero. If the charges aren't balanced, you must increase the number of cations and anions until the algebraic sum of the charges is zero.

If the charges are (2+) and (3-) the sum is (1-) and is not balanced. But the lowest common multiple of 2 and 3 is 6. So using subscripts increase the number of cations by 3 and the number of anions by 2.

Thus (2+) times 3 is (6+) and (3-) times 2 is (6-) and (6+) + (6-) is zero. Balanced.

If the cation has multiple charges, the proper charge is indicated by a Roman numeral placed after the cation's name. Remember to treat polyatomic ions as complete unit, do not separate them.

EXAMPLE #1: calcium chloride is CaCl2 (calcium 2+) (chlorine 1-) increase the chlorine to two.

EXAMPLE #2: lithium nitrate is LiNO3 (lithium 1+) (nitrate 1-) balanced.

EXAMPLE #3: iron (II) sulfate is FeSO4

Writing Molecular Formulas (covalent)

When writing the chemical formula for covalent compounds place the less electronegative element first followed by the more electronegative element and use subscripts to indicate the number of atoms present. Remember that prefixes are used only with covalent compounds.

EXAMPLE #1: carbon dioxide is CO2 (1 C) (2 O)

EXAMPLE #2: phosphorous pentachloride is PCl5 (1 P) (5 Cl)