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Formulas Combining symbols of different elements in a compound is called a chemical formula. Metal written first, nonmetal 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 Fe2O3 2 hydrogen atoms and 1 oxygen atom in one molecule of water 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: 1. Write down chemical symbols. 2. List oxidation number in the upper right hand corner of each symbol. 3. 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. 2. 3. 4. 5. 6. 7. 8. H+1 + S-2 ______________ Li+1 + F-1 ______________ Ca+2 + F-1 ______________ Ba+2 + Cl-1 ______________ K+1 + O-2 ______________ Na+1 + N-3 ______________ Li+1 + B-5 ______________ Ba+2 + F-1 ______________