1. Using ion models to determine chemical formulas
Chemical Practice
Using ion models to determine chemical formulas

2. How do the models work? Designed to show electron movement.
POSITIVE ions are trying to lose electrons so the model has an “arrow” showing electrons leaving.
NEGATIVE ions are trying to gain electrons so the model can “accept” electrons.
Arrow shows one electron leaving +1
Spaces show how many electrons the ion is trying to gain -2

3. EXAMPLES: Potassium (K) and Chlorine (Cl) **Write the ions first.
K+1 Cl-1
**Find the puzzle pieces representing each ion.
**One molecule of the compound is whatever it takes to complete the puzzle.
One of each element makes up the compound.
**Write the compound formula:
KCl +1 -1

4. Magnesium (Mg) and Chlorine (Cl)
Find the ions:
Find ion models:
Put puzzle pieces together to find ONE molecule of compound:
Write the formula:
Mg+2 Cl-1
MgCl2 -1 +2 -1 +2
Two
NEGATIVE
ions -1
One
POSITIVE
ion

5. Another way to determine the formula …
Write the ions in order: POSITIVE ion before NEGATIVE ion.
Mg+2 Cl-1
Cross over just the numbers of the charge.
Mg+2 Cl-1
Mg1Cl2
And, since the “1” is understood: MgCl2

6. Hydrogen (H) and Phosphate (PO4)
PO4  polyatomic ion
{These can be found on the back of your periodic table.}
Find each ion:
Put ion pieces together:
NOTE: You can only use the ions that make up the compound.
Write the formula:
Polyatomic ion = ion made of more than one atom
H+1 PO4-3
H3PO4 +1 -3 +1 +1

7. Again, you could determine the formula using the cross-over formula …
Write the ions in positive  negative order.
H+1 PO4-3
Cross over just the numbers of the charges.
H+1 PO4-3
H3PO41
Again, the “1” is understood and should not be included in the formula: H3PO4

8. Ammonium (NH4) and Phosphate (PO4)
Write the ions:
Combine the puzzle pieces:
How do you write more than one polyatomic ion?
Write compound formula:
NH4+1 PO4-3 +1 -3 +1 +1
Use parenthesis to show more than one polyatomic ion in a compound.
(NH4)3PO4

9. You could also use the cross-over method for finding the formula:
Write the ions in positive  negative order.
NH4+1 PO4-3
Cross over just the numbers of the charges.
(NH4)3PO41
Again, the “1” is understood and should not be included in the formula: (NH4)3PO4

10. Aluminum (Al) and Sulfate (SO4)
Write the ions:
Combine the puzzle pieces:
Remember that you can only use the ions that are part of the compound.
Write the formula:
Al+3 SO4-2 -2 +3 -2 +3 -2
Al2(SO4)3

11. Again, you could use the cross-over method for finding the formula:
Write the ions in positive  negative order:
Al+3 SO4-2
Cross over just the numbers of the charges.
Al2(SO4)3

12. Calcium (Ca) and Sulfate (SO4)
Write the ions:
Combine the puzzle pieces:
Notice that the molecule is complete with one of each ion.
Write the formula:
Ca+2 SO4-2 +2 -2
CaSO4

13. What if you use the cross-over method?
Write the ions in positive  negative order:
Ca+2 SO4-2
If you cross over just the numbers, you would have this formula: Ca2(SO4)2, but remember Ca is losing 2 electrons to sulfate so this doesn’t make sense.
If the numbers on the charge are identical, you will only need one of each: CaSO4