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.

• NEGATIVE ions are trying to gain electrons so the model can “accept” electrons.

+1 Arrow shows one electron

leaving

-2

Spaces show how many

electrons the ion is trying to gain

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

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

+2-1

+2-1

-1One

POSITIVEion

TwoNEGATIVE

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{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

-3

+1

+1

+1

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

NH4+1 PO4

-3

Ammonium (NH4) and Phosphate (PO4)

• Write the ions:• Combine the puzzle

pieces:

• How do you write more than one polyatomic ion?

• Write compound formula:

-3

+1

+1

+1

•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:

• Remember that you can only use the ions that are part of the compound.

• Write the formula:

Al+3 SO4-2

+3 -2

+3

-2

-2

Al2(SO4)3

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.

Al+3 SO4-2

Al2(SO4)3

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

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