Writing Chemical Formulas
Target 3.5: Derive the chemical formulas for
compounds
Writing the Chemical Formula
• Step one: Determine the charges
• Determine the charge of the cation
• How many electrons will it lose
• Determine the charge of the anion
• How many electrons will it gain
Writing the Chemical Formula
• Step Two: Determine how many
cations and anions are required to
make an electrically neutral
compound
Writing the Chemical Formula
• Step three: Lowest possible terms
• Just like with chemical equations
• MgO is correct
• Mg2O2 in not
Writing the Chemical Formula
• Example: Lithium Fluoride
• Step one: Determine charges
• Li+
• F-
• Step two: Determine how many cations
and anions are required to make an
electrically neutral compound
• It is a one to one ratio
• One positive Li+ and one negative F-
• Step three: Lowest possible terms
• LiF
Writing the Chemical Formula
• Example: Aluminum Chloride
• Step one: Determine charges
• Al3+
• Cl-
• Step two: Determine how many cations
and anions are required to make an
electrically neutral compound
• One Al has + + +
• Three Cl - Cl- Cl- are needed
• Step Three: Lowest Possible terms
• AlCl3
Writing the Chemical Formula
• Check the pattern in the charges
• Al3+
• Cl-
• Compared to the formula
• AlCl3