1. Ionic Compounds: Writing Formulas

2. Empirical Formulas = smallest whole-number ratio of elements
Ionic compounds: empirical formulas only

3. Identify the empirical formulas
H2O
H2O2
CaF2
NaCl
CH4
C2H6
FeCl3
Empirical
Molecular
Empirical (Ionic)
Empirical (Ionic)
Empirical
Molecular
Empirical (Ionic)

4. Binary Ionic Compounds
Composed of two different elements
Positive monatomic metal ion
Negative monatomic nonmetal ion
Note: binary compound may contain more than 2 ions but only 2 kinds of ions like Al2O3

5. Oxidation Number O-2 Ca+2 Na+1 Fe+2 and Fe+3 Charge of a monatomic ion
Some elements form only 1 ion
Some elements can form more than 1 ion
O-2
Ca+2
Na+1
Fe+2 and Fe+3

6. Oxidation State of Metal Elements
Group 1 ions always +1
Group 2 ions always +2
MOST group 13 ions are +3
Groups 14, 15, 16, and 17: multiple oxidation states
Transition metals (Group B) can have more than 1 oxidation state

7. Writing Formulas: Binary Ionic Compounds
first rule is:
POSITIVE ION FIRST
(metal)
How do you get subscripts?
start by writing the ions

8. Compounds are neutral:
Sum of all charges must = 0
(+) and (-) charges must = each other

9. Charges: Equal but Opposite
Na+1 and Cl-1: NaCl (-1) = 0
Mg+2 and O-2: MgO (-2) = 0
Al+3 and P-3: AlP (-3) = 0
Rule: write the symbols, (+) FIRST!

10. Try a few formulas: Li+1 and I-1 Ca+2 and O-2 Al+3 and N-3 K+1 and F-1
Ba+2 and S-2
LiI
CaO
AlN KF
BaS

11. Charges: Opposite but NOT equal
Criss-Cross Method:
Mg+2 and Cl-1, CROSS and DROP!
(# only, forget signs!)
Mg1Cl2 (if subscript is 1, forget it)
MgCl2 (1 Mg+2 and 2 Cl-1)
[check math: = 0]

12. Try a few formulas: Ca+2 + Cl-1 Na+1 + O-2 Cs+1 + S-2 Al+3 + Cl-1
Al+3 + Se-2
Mg+2 + F-1
K+1 + N-3
CaCl2
Na2O
Cs2S
AlCl3
Al2Se3
MgF2
K3N