1. Formulas
Combining symbols of different elements in a compound is called a chemical formula. Metal written first, non-metal written second.
Example: Na and Cl = NaCl (salt)
H and Cl = HCl (hydrochloric acid/stomach acid)
Fe and S = Fe2S3 (iron sulfide)
C and O = CO2 (carbon dioxide)
O3 = (Ozone)
What is the chemical formula for water?
Answer  H2O

2. Subscripts
Written below and after the symbol of the atom to indicate the number of atoms of that element in the compound.
if no subscript, then = 1 atom
Example: H2O 2 hydrogen atoms and 1 oxygen atom
in one molecule of water
Fe2O3 How many iron atoms? _____
Answer  2
How many oxygen atoms? ____
Answer  3

3. What is the ratio of the carbon atoms to the oxygen atoms in one molecule of carbon dioxide (CO2)? _________
Answer  1:2

4. CRISS CROSS METHOD Used to determine chemical formula:
Write down chemical symbols.
List oxidation number in the upper right hand corner of each symbol.
With number write down + for metals* and – for nonmetals.
4. Criss Cross oxidation numbers to new locations.
5. Drop + and -. Rewrite the formula.

5. Examples of Writing Formulas
1. H S-2  ______________
2. Li F  ______________
3. Ca F  ______________
4. Ba Cl  ______________
5. K O  ______________
6. Na N  ______________
7. Li B  ______________
8. Ba F  ______________