Writing Chemical Formulas
Ionic Chemical Formulas (contain a metal in the chemical formula)
Ionic Compounds with non-metals ending in –ide:
1. Determine the symbols for the elements involved from the chemical name.
2. Determine the positive charge of the metal from the periodic table. If there is a roman
numeral present, the charge of the metal is equal to the value of the roman numeral.
3. Determine the negative charge of the non-metal from the periodic table.
4. If the charges are…
a. equal, but opposite in value, the formula is written as it appears.
b. If the charges are not equal but are opposite in value, you must use the criss-cross
method. Ignoring the charge symbol, bring down the value diagonally from top to
bottom.
Examples:
Calcium Oxide
Ca2+ O2-
CaO (no criss-cross)
Sodium Chloride
Na1+ Cl1-
NaCl (no criss cross)
Iron (II) Nitride
Fe2+ N3-
Fe2N3(criss cross)
Potassium Sulfide
K1+S2-
K 2S (criss cross)
Ionic Compounds with non-metals ending in –ite or –ate:
1. Determine the symbols for the elements involved from the chemical name.
2. Determine the positive charge of the metal from the periodic table. If there is a roman
numeral present, the charge of the metal is equal to the value of the roman numeral.
3. Determine the negative charge of the polyatomic ion from the charge sheet.
4. If the charges are…
a. equal, but opposite in value, the formula is written as it appears.
b. If the charges are not equal but are opposite in value, you must use the criss-cross
method. Ignoring the charge symbol, bring down the value diagonally from top to
bottom. Place all polyatomic ions, if more than one is present, in parenthesis
to avoid confusion regarding the amount present.
Examples:
Lithium Nitrate
Li1+ NO31-
LiNO3
Magnesium Phosphite
Mg2+PO33-
Mg3(PO3)2
Gold (IV) Acetate
Au4+ C2H3O21-
Au(C2H3O2)4
Writing Chemical Formulas
Acids (compounds that have hydrogen as the first element)
If the name of the acid begins with prefix hydro-:
1. Write down H ion, this has a charge of 1+.
2. Change the –ic ending back to –ine. Find the non-metal and determine the charge of the nonmetal utilizing the periodic table.
3. If the charges are…
a. equal, but opposite in value, the formula is written as it appears.
b. If the charges are not equal but are opposite in value, you must use the criss-cross
method. Ignoring the charge symbol, bring down the value diagonally from top to
bottom.
Examples:
Hydroiodic acid
iodic = iodine
H1+ I1-
HI
Hydrochloric acid
chloric = chlorine
H1+Cl1-
HCl
If the name of the acid does not begin with hydro-:
1. Write down H ion, this has a charge of 1+.
2. Change the –ic ending back to –ate. Change the –ous ending back to –ite. Find the ion
present and the charge of the ion present using the ion sheet.
3. If the charges are…
a. equal, but opposite in value, the formula is written as it appears.
b. If the charges are not equal but are opposite in value, you must use the criss-cross
method. Ignoring the charge symbol, bring down the value diagonally from top to
bottom. Place all polyatomic ions, if more than one is present, in parenthesis to
avoid confusion regarding the amount present.
Examples:
Nitric Acid
nitric = nitrate
H1+NO31-
HNO3
Carbonic Acid
carbonic = carbonate H1+ CO32-
H2CO3
Phosphorous Acid
phosphorous = phosphite
H1+PO33-
Covalent Compounds (does not contain metal in formula)
Write the compounds based on the numerical prefixes present.
Example:
Carbon monoxide
CO
Boron trifluoride
BF3
Dinitrogen tetrabromide
N2Br4
H3PO3