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Writing Chemical Formulas—Criss Cross Method
Now that we have looked at how compounds trade electrons, wouldn’t it be nice if there was a quicker and simpler
way of determining the formula that does not involve puzzle pieces? Relax—there is!! It is called the “criss-cross”
method. Here’s how it works.
1. Look at the name and determine the ions in the compound by looking at their placement in the periodic table or
the Table of Common Oxidation Numbers on the back of the periodic table you were given. The cation ion
(positive ion) is written first, then the anion (negative ion).
2. Write the ion symbol, and then write the oxidation number of the ion to the upper right of the symbol. You’ll find
the oxidation number listed at the top of each column.
3. Draw a box around the oxidation number. The box will make life easier for you later!
4. Now, look at the two oxidation numbers. If they add up to zero, you are done. Just re-write the symbols of the
ions without the oxidation numbers (remember to write the cation first, then the anion).
5. If the two oxidation numbers don’t add up to zero, take the oxidation number of the cation (without the + sign)
and place only the number as a subscript after the anion. Now take the oxidation number of the anion (without
the - sign) and place only the number as a subscript after the cation. Re-write the symbols of the ions this time
without the oxidation numbers. If the subscript is a one (1) you do not need to write it down.
Sometimes, these ions themselves are made of more than one atom or element. We called these ions polyatomic (many
atoms). Treat them as a single ion. Whenever you write a polyatomic ion, always put the entire ion in parenthesis. If you
need a subscript, write it outside the parenthesis.
1. sodium chloride
Na Cl Add the charges: +1 + -1 = 0 done
Write the Formula: NaCl
2. magnesium phosphide
Mg P Add the charges: +2 + -3 = -1 go to step 5 & criss cross
Write the Formula: Mg 3 P 2 (notice the subscripts)
3. zinc sulfide
Zn S Add charges:
4. copper II fluoride
Cu F Add charges:
Elements in the d and f blocks can have more than one oxidation number. We use roman numbers to indicate the
oxidation number: Examples: Cu I is +1, and Cu II is +2