1. Writing Chemical
Formulas

2. Chemical Formulas represent compounds.

3. Oxidation Numbers are used to determine the ratio in which elements combine to form compounds.

4. Understanding Chemical Formulas
Chemical formulas are composed of a positive half and a negative half.
Ex. - Water is a compound you know to have a formula of H2O.

5. What about Ions? Cations Positively Charged Atoms Na+
Anions Negatively Charged Atoms
Cl-

6. The element with the positive oxidation number is always written first.
The element with the negative oxidation number is always written second.

7. The total of the oxidation numbers in a compound must equal zero.+1 -2
Hydrogen’s oxidation number is +1 and oxygen’s is -2. With one H and one O, the total is not 0, it is -1!!!

8. Subscripts, small numbers to the lower right of the chemical symbol, represent the number of atoms for each element present in the compound.
The subscript of 1 is never written in a chemical formula. It is understood since the chemical symbol is there.
Add subscripts after a chemical symbol, when needed, to make the oxidation numbers total zero.
H2O

9. How to check if the formula is correct:
Multiply subscript by oxidation number for the total oxidation number of each element in a formula.
For Hydrogen:
(oxidation number +1)(subscript 2) = +2 total
For Oxygen:
(oxidation number -2)(subscript 1) = -2 total
The formula H2O is the correct formula!!!

10. Ca Cl There MUST be an easier way....and there is!! +2 -1
The easiest way to think of writing chemical formulas is to use the oxidation number (without the + or -) of one element as the subscript of the other element.
Ca Cl +2 -1

11. Cross over the oxidation numbers without the charges!!!+2 -1
Ca Cl

12. Ca Cl 2
REMINDER: DO NOT write a subscript of 1. Reduce the subscripts if needed.

17. Writing Formulas for Ionic Compounds
Find oxidation number for each element.
Check if charges are equal to zero.
Use criss-cross method if charges are NOT equal to zero.
Write symbol and correct subscript.

18. Writing Covalent Formulas
The prefixes tell you the subscript of each element within the formula
Mono - 1
Di - 2
Tri - 3
Tetra - 4
Penta - 5
Hexa - 6
Hepta - 7
Octa - 8
Nona - 9
Deca - 10

19. Steps for Writing Covalent Formulas
Write the symbol for both nonmetals.
Write the prefix for each element as a subscript.
If there is only one atom present for the 1st element DO NOT use a subscript.
If there is more than one atom present then write it as a subscript.

20. Write formulas for these
diphosphorus pentoxide
tetraiodide nonoxide
sulfur hexafluoride
nitrogen trioxide
Carbon tetrahydride
phosphorus trifluoride