Download Formula Notes and Criss Cross Method

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  ______________