Chemical Practice
Using ion models to determine
chemical formulas
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.
Arrow shows
+1
one electron
leaving
• NEGATIVE ions are trying to gain
electrons so the model can “accept”
electrons.
Spaces show
how many
electrons the ion
is trying to gain
-2
EXAMPLES:
Potassium (K) and Chlorine (Cl)
**Write the ions first.
K+1
Cl-1
**Find the puzzle pieces representing each ion.
+1
-1
**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
Magnesium (Mg) and Chlorine (Cl)
• Find the ions:
• Find ion models:
Mg+2
Cl-1
-1
+2
• Put puzzle pieces
together to find ONE
molecule of
compound:
One
POSITIVE
ion
• Write the formula:
-1
+2
-1
MgCl2
Two
NEGATIVE
ions
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
Hydrogen (H) and Phosphate (PO4)
• PO4  polyatomic ion Polyatomic ion = ion
{These can be found on the back
made of more than
of your periodic table.}
one atom
• Find each ion:
H+1
PO4-3
• Put ion pieces
+1
together:
-3
• NOTE: You can only
+1
use the ions that
+1
make up the
compound.
• Write the formula:
H3PO4
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
Ammonium (NH4) and Phosphate (PO4)
• Write the ions:
• Combine the puzzle
pieces:
NH4+1
PO4-3
+1
+1
-3
+1
• How do you write more
than one polyatomic
ion?
• Write compound
formula:
•Use parenthesis to
show more than one
polyatomic ion in a
compound.
(NH4)3PO4
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+1
PO4-3
(NH4)3PO41
• Again, the “1” is understood and should
not be included in the formula: (NH4)3PO4
Aluminum (Al) and Sulfate (SO4)
• Write the ions:
• Combine the puzzle
pieces:
Al+3
SO4-2
+3
• Remember that you
can only use the ions
that are part of the
compound.
-2
-2
+3
-2
• Write the formula:
Al2(SO4)3
Again, you could use the crossover method for finding the formula:
• Write the ions in positive  negative
order:
Al+3
SO4-2
• Cross over just the numbers of the
charges.
Al+3
SO4-2
Al2(SO4)3
Calcium (Ca) and Sulfate (SO4)
• Write the ions:
• Combine the puzzle
pieces:
Ca+2
SO4-2
+2
• Notice that the
molecule is complete
with one of each ion.
• Write the formula:
-2
CaSO4
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