Making Ionic Formulas
Write your answers on a separate sheet of paper.
Purpose: During this activity, use the models to construct neutrally charged ionic compounds.
Below is a diagram of the three cations (monovalent, divalent, trivalent) and the three anions (also
monovalent, divalent, and trivalent. A monovalent ion has a charge of plus or minus 1, divalent has
a charge of plus or minus 2 and trivalent has a charge of plus or minus charge.
CATIONS
+
+
+
+
ANIONS
+
-
-
-
-
-
+
Procedure:
1. Remove the models from your baggie. Arrange the pieces to fit the pattern shown above.
Each piece will represent a generic ion. For example the +1 ion could be K1+, or NH4 1+, and
the -1 ion could represent Cl1-, or NO3 1-.
2. Assume you have a generic +1 ion (X1+) joined to a generic -1 negative ion (Y1-). Using your
models make the appropriate pieces fit together in lock-and-key manner. Make the positive ion be
on the left side and the negative ion on the right.
Follow-up Questions:
Q1. Is the resulting compound neutrally charged?
Q2. Write the formula for the compound involving (X1+) and (Y1-). Are the charges still shown?
Why or why not? (Remember achieving neutrality is the goal.)
Q3. Write the formula for the following pairs of monovalent ions. (The positive ion is written first.)
A) Li1+ and Br1B) Cs1+ and F1- C) H1+ and Cl1In question three all the monovalent ions were monatomic which means each ion has just one atom.
Some monovalent ions are polyatomic. Here are seven polyatomic ions we have to memorize:
NH4 1+ (ammonium), OH1- (hydroxide), NO3 1- (nitrate), C2 H3 O2 1- (acetate), (carbonate) CO3 2-,
(sulfate) SO4 2- and (phosphate) PO4 3-. To write formulas using polyatomic ions you cannot change
the subscripts of the polyatomic ion itself. What is important is to balance the charges of positive
and negative ions.
Q4. Write the formula for the following monovalent ions (with polyatomic ions present).
A) Na1+ and NO3 1B) NH4 1+ and Br1C) K1+ and OH1-
D) NH4 1+ and NO3 1+
Back to the Procedure:
3. Now use a +2 ion piece and cause it to join with -1 ion piece(s).
Think to remember how many positive and negative pieces were needed to achieve neutrality.
4. Next use a +2 ion piece and cause it to join with a -2 ion.
Make a mental note how many positive and negative pieces were needed to achieve neutrality.
5. Finally use a +2 ion piece(s) and cause it to join with -3 ion piece(s).
Make a mental note how many positive and negative pieces were needed to achieve neutrality.
Follow-up Questions:
Q5. In each case neutrality has to be accomplished. Write the formula for three generic compounds
using X2+, and Y1-, Y2-, and Y3- respectively. Think about how many positive and negative pieces
would be needed to achieve neutrality
Q6. Complete the following table by writing the needed formula.:
Divalent cation
(X2+)
monovalent anion (Y1-) divalent anion (Y2-)
Cl1S2-
trivalent anion (Y3-)
N3-
Ba2+
Ca2+
There are also polyatomic ions that have -1, -2, and -3 charges. Notice that ammonium is the only
common positive polyatomic ion. Here are three more polyatomic ions you need to memorize.
Write these down and memorize both the formula and name: carbonate CO3 2-, sulfate SO4 2-, and
phosphate PO4 3-. When you use a polyatomic ion to write a formula simply write the entire formula
(of course since the charge has been neutralized do not list the charge.) If more than one polyatomic
ion is needed to write an neutrally charged formula in making the compound then enclose the
polyatomic ion in parentheses and write the appropriate subscript to the right of the parentheses.
For example Ca2+ forms compounds with OH1-, CO3 2-, and PO4 3- to form compounds
Ca(OH)2 , CaCO3 and Ca3 (PO4 ) 2 respectively. If a polyatomic ion is enclosed with parentheses it
says that more than one ion is needed and the number needed is expressed as a subscript to the right
of the parenthesis.
Q7. Complete the following table by writing the correct formula:
Divalent cation
(X2+)
monovalent anion (Y1-) divalent anion (Y2-)
NO31SO42-
Mg2+
Zn2+
Cu2+
Back to the Procedure:
6. Now use a -2 ion piece and cause it to join with +1 ion piece(s).
trivalent anion (Y3-)
PO43-
Make a mental note, how many positive and negative pieces were needed to achieve neutrality.
7. Next use a -2 ion piece and cause it to join with a +2 ion.
Make a mental note, how many positive and negative pieces were needed to achieve neutrality.
8. Finally use a -2 ion piece(s) and cause it to join with +3 ion piece(s).
Make a mental note, how many positive and negative pieces were needed to achieve neutrality.
Follow-up Questions:
Q8. In each case neutrality has to be accomplished. Write the formula for the generic compounds
using Y2-, and X1+,X2+, and X3+ respectively. Think about how many positive and negative pieces
would be needed to achieve neutrality. THE POSITIVE ION IS STILL WRITTEN FIRST.
Q9. Complete the following table by writing the needed formula. THE POSITIVE ION IS
WRITTEN FIRST.
Divalent anion
(Y2-)
monovalent cation(X1+) divalent anion (X2+)
K1+
Mg2+
trivalent anion (X3+)
Al3+
S2O2Q10. Complete the following table by writing the correct formula involving polyatomic ions (and
potentially the use of parentheses). THE POSITIVE ION IS ALWAYS WRITTEN FIRST.
Divalent anion
(Y2-)
monovalent cation(X1+) divalent anion (X2+)
NH41+
Ca2+
trivalent anion (X3+)
Sc3+
CO32SO42Back to the Procedure:
9. Now use a +3 ion piece and cause it to join with -1 ion piece(s).
Think to remember how many positive and negative pieces were needed to achieve neutrality.
10. Next use a +2 ion piece and cause it to join with a -2 ion piece(s).
Make a mental note, how many positive and negative pieces were needed to achieve neutrality.
11. Finally use a +2 ion piece(s) and cause it to join with a -3 ion piece(s).
Make a mental note, how many positive and negative pieces were needed to achieve neutrality.
Follow-up Questions:
Q11. In each case neutrality has to be accomplished. Write the formula for three generic compounds
using X3+, and Y1-, Y2-, and Y3- respectively. Think about how many positive and negative pieces
would be needed to achieve neutrality
Q12. Complete the following table by writing the needed formula.:
monovalent anion (Y1-) divalent anion (Y2-)
I1Se2-
Divalent cation
(X3+)
trivalent anion (Y3-)
N3-
Al3+
Fe3+
Q13. Complete the following table by writing the correct formula involving polyatomic ions and
potentially the use of parentheses.
monovalent anion (Y1-) divalent anion (Y2-)
C2H2O31SO42-
Divalent cation
(X3+)
trivalent anion (Y3-)
PO43-
Co3+
Sc3+
Cr3+
Q14. Complete the following table:
Cl1-
SO42-
NH41+
Mg2+
Al3+
Na1+
Be sure to return all the pieces to the baggie.
PO43-
O2-