1. Writing Ionic Formulas

2. Ions: Ca+2 O-2 Formula: CaO Ions: Al+3 N-3 Formula: AlN
There is an easy way to determine the ratio of an ionic formula when you are given the ions:
1. If the charges of the ions are the same, then the ratio is 1:1.
Ions: Ca+2 O-2
Formula: CaO
Ions: Al+3 N-3
Formula: AlN

3. Ions: Mg+2 Cl- Formula: MgCl2 Ions: K+ S-2 Formula: K2S
2. If the charges of the ions are different, then the ratio can be determined by
criss-crossing the charges.
Ions: Mg+2 Cl-
Formula: MgCl2
Ions: K+ S-2
Formula: K2S

4. Ions: Mg+2 Cl- Formula: MgCl2
Notice that we write the charge as a superscript with a + or – sign.
Ions: Mg+2 Cl-
But notice that the ratios are written as a subscript without any signs.
Formula: MgCl2

5. Ions: Al+3 Br- Formula: AlBr3
This is why we say that we criss-cross the numbers. We don’t just move them over, we also move them down!
Ions: Al+3 Br-
Formula: AlBr3

6. For each combination of ions, write the formula for the compound they would make.
(Be careful to make sure that if they have the same charge, you put them in a 1:1 ratio!) HF
K2O
MgCl2
CuI2
BaO
CaCl2
ZnI2
SrF2
MgO
FeBr2
Fe2O3
SrS

7. There are also ions that form after elements have shared electrons
There are also ions that form after elements have shared electrons. These ions are known as polyatomic ions, and there is one special rule when you write them in a formula.
SO4-2
CO3-2
NH4+
NO3-
OH-
PO4-3

8. This is NO DIFFERENT than before!
1. If the charges of the ions are the same, then the ratio is 1:1.
Ions: Ca+2 SO4-2
Formula: CaSO4
Ions: Al+3 PO4-3
Formula: AlPO4
This is NO DIFFERENT than before!

9. Ions: Mg+2 OH- Formula: Mg(OH)2 MgOH2
2. If the charges are different, then the ratio can be determined by criss-crossing the charges.
2. If the charges are different, then the ratio can be determined by criss-crossing the charges and add parentheses only if we need more than one polyatomic ion.
Ions: Mg+2 OH-
Formula:
Mg(OH)2
MgOH2
Problem: The formula looks like we have 1 Mg, 1 O, and 2 H’s when we really meant to write that we had 2 OH’s. To fix this, we add parentheses around the polyatomic ion.

10. Notice that we only need parentheses around the polyatomic ions when there is more than one in the formula.
Ions: Ca SO4-2
Ions: Al SO4-2
Formula: CaSO4
Formula: Al2(SO4)3
Ions: Li NO3-
Ions: Mg NO3-
Formula: LiNO3
Formula: Mg(NO3)2
Ions: K PO4-3
Ions: Sr PO4-3
Formula: K3PO4
Formula: Sr3(PO4)2

11. It is important that you are aware that any ion with more than one CAPITAL letter has more than one element and is a polyatomic ion.
Ions: Na OH-
Ions: Al OH-
Formula: NaOH
Formula: Al(OH)3
Ions: H CN-
Ions: Ca CN-
Formula: HCN
Formula: Ca(CN)2

12. (Be sure to use parentheses appropriately!)
For each combination of ions on your notes, write the formula for the compound they would make.
(Be sure to use parentheses appropriately!)
Ca(OH)2
Ba(NO3)2
Al2 (CO3) 3
NH4Cl
Mg(CN)2
Ca(NO3) 2
Fe2 (SO4)3
Zn(NO3) 2