2.3 Writing Formulas for Chemical Compound
MAKE SURE YOU HAVE A PERIODIC TABLE IN FRONT OF YOU AT ALL TIMES IN
THIS SECTION
Identifying Binary Ionic Compounds
 They end in “-ide” (except “hydroxide and cyanide”) & DO NOT contain covalent prefixes
Writing formulas for Binary Ionic Compounds (metal element + non-metal element)
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Write the symbol and charge for the cation (metal).
Write the symbol and charge for the anion (non-metal).
The sum of all of the positive charge and all of the negative charge equals zero in a neutral atom.
If the total charge is not equal to zero, you must adjust the number of cations or anion to create a
neutral compound.
Use subscripts to show how many of each type of ion is there
Note: Use your periodic table to help you determine the charges of most of the common cations and anions.
Examples:
Sodium chloride
metal (sodium) symbol & charge:
nonmetal (chlorine) symbol & charge:
Sum of the positive & negative charges:
Formula:
If we combined one of each ion we would write: NaCl. Adding up the charges would give an overall charge of zero
since algebraically 1+-1 = 0. Since only one of each ion is needed we do not need any subscripts.
Calcium bromide
Metal (calcium) symbol and charge:
Nonmetal (Bromine) symbol and charge:
Sum of the positive & negative charges:
Formula:
If we combined one of each ion we would write: CaBr . Adding up the charges would give an overall charge of +1
since algebraically 2+-1 = +1. An additional bromide ion is needed to make the charges equal to zero so a subscript
of 2 is placed under the bromide ion.
The Criss Cross Method is a simple way to determine the subscripts
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Write the symbol and charge for the cation (metal).
Write the symbol and charge for the anion (non-metal).
Cross ONLY the charge numbers down diagonally as subscripts IF they are not equal to each other
Simplify subscripts if you can
Ca +2 Br-1 
SELF CHECK!:
Compound name
Cation symbol & Charge
Anion Symbol &
charge
Formula
Cesium chloride
Potassium oxide
Calcium sulfide
Lithium nitride
Writing Formulas for Ternary Ionic Compounds
 They DO NOT end in “-ide” (except “hydroxide and cyanide”) & DO NOT contain covalent prefixes
 The same process is used as for binary ionic compounds except the anion will be a polyatomic ion that has its own
charge. (The exception is ammonium NH4 + which will be a cation.)
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Examples:
Write the symbol & charge for the cation and anion
Adjust the number of each ion so that a neutral compound is formed by adding subscripts to indicate
how many of each ion are needed if more than one.
If you need more than 1 of the polyatomic ions, you must place parentheses around the formula for
the polyatomic ion BEFORE you subscript it. The subscript goes outside the parenthesis. YOU DO
NOT WANT 2 SUBSCRIPTS SIDE BY SIDE---MUST BE SEPARATED BY THE PARENTHESIS.
Shortcut Rule can work as well.
Sodium Carbonate
Metal (Sodium) symbol & charge:
Polyatomic ion (carbonate) symbol & charge:
Sum of the positive & negative charges:
Formula:
Magnesium nitrate
Metal (magnesium) symbol & charge:
Polyatomic Ion (nitrate) symbol & charge:
Sum of the positive & negative charges:
Formula:
SELF CHECK!
Sodium nitrate
Calcium chlorate
Potassium sulfite
Calcium hydroxide
Writing Formulas for Ionic Compounds with a Transition Metal (type II metal)
 These compounds will have roman numerals.
 The same rules apply as for the compounds above. The Roman numeral gives you the charge of the transition
metal).
Examples: Iron (III) oxide
Type II metal (iron) symbol & charge:
Nonmetal (oxygen) symbol & charge:
Sum of the positive & negative charges:
Formula:
Copper (II) nitrate
Type II metal (copper) symbol & charge:
Polyatomic Ion (nitrate) symbol & charge:
Sum of the positive & negative charges:
Formula:
SELF CHECK!
Iron (II) nitrate
Copper (I) chloride
Lead (IV) hydroxide
Tin (II) oxide
Writing Formulas for BINARY COVALENT COMPOUNDS
These compounds contain prefixes in their names.
 To write these formulas: The prefixes indicate how many of each atom that you need. If there is not a prefix is it
understood to be 1. YOU DO NOT HAVE TO WORRY ABOUT CHARGES FOR THESE. NO CRISS CROSS METHOD
EITHER!
Examples:
dinitrogen tetraoxide
Silicon dioxide
SELF CHECK!
Carbon monoxide
Sulfur tetraiodide
Trichlorine pentasulfide
Mixed Practice: Write the formulas for the following chemicals
Magnesium hydroxide
Copper (II) nitrate
Iron (III) oxide
Nitrogen dioxide
Sodium bicarbonate