1. Ionic Compounds: Writing Formulas

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

3. Identify the empirical formulas
H2O
H2O2
CaF2
NaCl
CH4
C2H6
NiO
FeCl3
Empirical – NOT ionic
Molecular – NOT ionic
Empirical, Ionic
Empirical, Ionic
Empirical – NOT ionic
Molecular – NOT ionic
Empirical, Ionic
Empirical, Ionic

4. Formula Unit = simplest ratio of ions in ionic compound Examples:
KBr is formula unit
potassium ions & bromide ions are in a 1-to-1 ratio
AlCl3 is formula unit
Aluminum & chloride ions in 1-to-3 ration

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

6. Oxidation Number or State
Na+1
charge of monatomic ion
has right superscript
Some elements form only 1 ion
Some elements can form more than 1 ion
Ca+2
O-2
Al+3 and Ag+1
Fe+2 and Fe+3

7. Oxidation States Group 1 ions always +1 Group 2 ions always +2
MOST group 13 ions are +3
Groups 14, 15, 16, and 17 have multiple oxidation states
both (+) and (-)
Transition metals (Group B) can have more than 1 oxidation state
but are always (+)

8. Writing Formulas for Binary Ionic Compounds
first rule in writing formulas for ionic compounds is
POSITIVE ION FIRST
But how do you get the subscripts?
start by writing formulas from the ions

9. Compounds are electrically neutral
Sum of all charges in compound
should be 0
(+) and (-) charges must = each other

10. Equal but Opposite Charges
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 symbols, (+) FIRST!

11. 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

12. Criss-Cross Method If charges are not equal but opposite:
Mg+2 and Cl-1, CROSS and DROP!
(# only, forget signs!)
Mg1Cl2 but if subscript is 1, forget it!
MgCl2 means 1 Mg+2 and 2 Cl-1

13. Check the Math MgCl2 means 1 Mg+2 and 2 Cl-1
(1)(+2) + (2)(-1) = = 0
charges MUST add up to zero!

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

15. Of course, it gets more difficult
Potassium (K) and Fluorine (F)
Zinc (Zn) and Iodine (I)
Sodium (Na) and Oxygen (O)
Magnesium (Mg) and Oxygen (O)
Aluminum (Al) and Oxygen (O)
Calcium (Ca) and Bromine (Br)
Cesium (Cs) and Iodine (I)
Silver (Ag) and Sulfur (S) KF
ZnI2
Na2O
MgO
Al2O3
CaBr2
CsI
Ag2S