Writing Chemical Formulas for Ionic Compounds

Name: ______

This is not a worksheet –it is your “notes” for formula writing when 2 ions form an ionic compound.

Writing Chemical Formulas For Ionic Compounds

Understanding what you are “given”:

-A correct formula for any ionic compound that you are given this year will contain 1 type of cation and 1 type of anion. The cation’s name will come first; the anion’s name will come second.

Cations:

-If you can’t find the cation’s name on the periodic table – you might have a “poly-atomic” cation called “ammonium”. Watch for it! All other cation’s come directly from an element’s name on the periodic table.

-If a cation has a (I), (II), or (III), etc. after its name...You are being given its charge! “Cations” by definition are all positively charged, since they form when metallic elements lose electrons: (I) = +1. (II) = +2. (III) = +3.

Actually, a chemist writes the “+” after the number: 1+, 2+, 3+.

-If a cation doesn’t have a Roman Numeral after it… You need to find its charge from the periodic table. You should circle the cations’ charges on the periodic table today. The number you circle tells you the positive charge of the cation.

-Many (but not all) of the elements in the “middle” of the periodic table have multiple charges – that is when (and why) the Roman numerals are used – to identify the specific charge the cation has in the given compound.

Anions:

-Nonmetallic elements (found to the right side of the “staircase line” on the periodic table) form anions by gaining electrons. So, every anion is negatively charged. The periodic table you were given is confusing – So, you should write the anion charges in by hand today.

Halogens (the group 7 elements) = 1—. Gr 6 nonmetals = 2—. Gr 5 nonmetals = 3—.)

Names of the anions:

-The ending (often the last syllable) of the nonmetallic elements on the periodic table is changed and is replaced with “ide”. So, if something “sounds like an element but isn’t”, it’s probably an anion from the nonmetal section of the periodic table!

-If an anion’s name is not from the nonmetal section of the periodic table, then you have a “polyatomic anion” – whose name and formula is given to you for use during the test on a “test resource” sheet.

How do I use the charges of a cation and anion when I write an ionic compound’s formula?

-By “dropping, swapping, and simplifying” (or, “drop, swap, reduce”). This is a method that gives you a correct “neutralized” formula. [The sum of the positive charges exactly cancels out with the sum of all the negative charges.] After both “dropping and swapping”, the positive charge appears as the anion’s subscript (the lower right # in the formula); and, the negative charge appears as the cation’s subscript. “Simplifying” means, whenever possible, reduce the two (dropped/swapped) subscripts.