1. Chemical Formula
Tells the relative number of atoms of each element in a compound
Molecular: (covalent bonding)
Formula represents the number of atoms of each element in a single molecule
Ionic: (ionic bonding)
Formula represents the simplest ratio of positive and negative ions in one formula unit
Episode 601

2. Ionic Bond
The attraction that binds opposite charged ions together.
Metals
Give away electrons
Ions are positively charged
Cations
Non-metals
Take electrons
Ions are negatively charged
Anions Al +3 Cl -1
AlCl3
Episode 601

3. Writing Ionic Formulas
Write the symbols for the elements
The cation is written first (metal)
The anion is written second (non-metal)
Determine the charge on each ion
Select subscript that will make the total positive charge equal to the total negative charge.
sodium chloride
magnesium chloride +1 -1 +2 -1 Na Cl Mg Cl
NaCl
MgCl2
Episode 601

4. Criss-Cross Method for Formula Writing
Write the symbol of cation followed by symbol of anion along with their ionic charges.
Use the absolute value of the charge of each ion as the subscript for the other.
If the subscript is one, omit it.
If the subscripts are the same, omit them.
Subscripts must be simplified in an ionic formula.
barium iodide
calcium sulfide
Ba+2
I-1
Ca+2
S-2
Ba1 I2
Ca2 S2
BaI2
CaS
Episode 601

5. Polyatomic Ions in formula writing
Use the same steps to write formula
Treat the polyatomic ion as a single ion
If the subscript for the polyatomic is more than 1, put polyatomic ion in ( ) with subscript on the outside.
sodium nitrate
calcium nitrate
Na+1
NO3-1
Ca+2
NO3-1
Na1
(NO3)1
Ca1
(NO3)2
NaNO3
Ca(NO3)2
Episode 601

6. Examples: Calcium Oxide Potassium chlorate Aluminum sulfate CaO KClO4
SO4-2
Al2(SO4)3
Al2
(SO4)3
Episode 601