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

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

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

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.

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