Forming Ionic
Compounds
Adrena McDonald
Venture Alternative High School
Before We Begin
• Cut out the ions from your student
worksheets.
• Have a glue stick ready to attach the
ions to your compound worksheet.
• Now, let’s do the first few together.
1. Boron (B+3) + Chlorine (Cl-1)
1 boron atom for every 3 chlorine atoms
1. BCl3
Cl-1
B+3
Cl-1
Cl-1
boron chloride
BCl3
2. Magnesium (Mg+2) + Chlorine (Cl-1)
1 magnesium atom for every 2 chlorine atoms
2. MgCl2
Cl-1
Mg+2
Cl-1
magnesium chloride
MgCl2
3. Sodium (Na+1) + Chlorine (Cl-1)
1 sodium atom for every 1 chlorine atom
3. NaCl
Cl-1
Na+1
sodium chloride
NaCl
4. Calcium (Ca+2) + Sulfur (S-2)
1 calcium atom for every 1 sulfur atom
4. CaS
Ca+2
S-2
calcium sulfide
CaS
5. Aluminum (Al+3) + Nitrogen (N-3)
1 aluminum atom for every 1 nitrogen atom
5. AlN
Al+3
N-3
aluminum nitride
AlN
Now, it’s your turn.
Do the remaining questions
on your own.
6. Calcium (Ca+2) + Bromine (Br-1)
1 calcium atom for every 2 bromine atoms
6. CaBr2
Br-1
Ca+2
Br-1
calcium bromide
CaBr2
7. Potassium (K+1) + Chlorine (Cl-1)
1 potassium atom for every 1 chlorine atom
7. KCl
K+1
Cl-1
potassium chloride
KCl
8. Boron (B+3) + Oxygen (O-2)
2 boron atoms for every 3 oxygen atoms
8. B2O3
O-2
B+3
O-2
boron oxide
+3
B
BO
2
3
O-2
9. Magnesium (Mg+2) + Oxygen (O-2)
1 magnesium atom for every 1 oxygen atom
9. MgO
Mg+2
O-2
magnesium oxide
MgO
10. Aluminum (Al+3) + Fluorine (F-1)
1 aluminum atom for every 3 fluoride atoms
10. AlF3
F-1
Al+3
F-1
F-1
aluminum fluoride
AlF3
11. Potassium (K+1) + Iodine (I-1)
1 potassium atom for every 1 iodine atom
11. KI
I-1
K+1
potassium iodide
KI
12. Sodium (Na+1) + Nitrogen (N-3)
3 sodium atoms for every 1 nitrogen atom
12. Na3N
Na+1
N-3
Na+1
Na+1
sodium nitride
Na3N
13. Potassium (K+1) + Bromine (Br-1)
1 potassium atom for every 1 bromine atom
13. KBr
Br-1
K+1
potassium bromide
KBr
14. Lithium (Li+1) + Fluorine (F-1)
1 lithium atom for every 1 fluorine atom
14. LiF
F-1
Li+1
lithium fluoride
LiF
15. Magnesium (Mg+2) + Nitrogen (N-3)
3 magnesium atoms for every 2 nitrogen atoms
15. Mg3N2
Mg+2
N-3
Mg+2
N-3
Mg+2
magnesium nitride
Mg3N2
Questions?
•
Boron (B+3) + Chlorine (Cl-1)  boron chloride
You can also determine the formula for a compound using the
“criss-cross” method. Take the numbers from the ions’
superscripts and cross them to become the subscripts in the
compound. Look at question #1.
3
+3
B
1
-1
Cl
B 1 Cl 3
BCl3
•
Magnesium (Mg2+) + Nitrogen (N-3) Magnesium nitride
• Look at #15 to practice the “criss-cross” method again.
2
+2
Mg
3
-3
N
Mg
N
3
Mg3N2
2
•
Potassium (K+1) + Sulfate (SO4-2)  potassium sulfate
•
This also works for polyatomic ions.
1
+1
K
K
(SO4
2
-2
)
(SO4)
K2SO4
Parentheses are not used unless a subscript is
needed after the polyatomic ion, as you will see in
the next example.
•
Aluminum (Al+3) + Sulfate (SO4-2) aluminum sulfate
• Use parentheses to indicate that more than one of the same
polyatomic ion is included in the compound.
3
+3
Al
(SO4
2
-2
)
Al (SO4)
Al2(SO4)3
• If the superscript charges add to zero, DO
NOT criss-cross. All compounds have a
zero charge, so it takes one of each ion to
make the compound:
• Ca+2 + O-2  CaO
(+2 and -2 = 0)
• B+3 + N-3  BN
(+3 and -3 = 0)
Released TAKS Question
calcium (Ca+2) + chlorine (Cl-1)  calcium chloride
The chemical formula for calcium chloride is --A)
B)
C)
D)
Ca2Cl
CaCl
CaCl2
Ca2Cl3
2
+2
Ca
1
-1
Cl
Ca 1 Cl 2
CaCl2
Released TAKS Question
Al3+ SO42According to this information, what is the chemical formula
for aluminum sulfate?
A)
B)
C)
D)
AlSO4
Al2(SO4)3
Al3(SO4)2
Al6SO4
3
+3
Al
2
-2
)
(SO4
Al (SO4)
Al2(SO4)3
Questions?
• Good job!!!
• Don’t forget to put your name on your
paper before you turn it in.
Ca+2
Li+1
O-2
Na+1
Mg+2
Na+1
F-1
O-2
Ca+2
Na+1
K+1
F-1
S-2
Br-1
Ca+2
N-3
Mg+2
N-3
Mg+2
Al+3
Al+3
K+1
P-3
S-2
Br-1
Br-1
B+3
I-1
Mg+2
K+1
Cl-1
Cl-1
Cl-1
F-1
O-2
P-3
B+3
Cl-1
Write the chemical formula for the following compounds. Place the cutouts in the
corresponding boxes on the following pages.
1. Boron (B+3) + Chlorine (Cl-1)  boron chloride
____________
2. Magnesium (Mg+2) + Chlorine (Cl-1)  magnesium chloride ____________
3. Sodium (Na+1) + Chlorine (Cl-1)  sodium chloride
____________
4. Calcium (Ca+2) + Sulfur (S-2)  calcium sulfide
____________
5. Aluminum (Al+3) + Nitrogen (N-3)  aluminum nitride
____________
6. Calcium (Ca+2) + Bromine (Br-1)  calcium bromide
____________
7. Potassium (K+1) + Chlorine (Cl-1)  potassium chloride
____________
8. Boron (B+3) + Oxygen (O-2)  boron oxide
____________
9. Magnesium (Mg+2) + Oxygen (O-2)  magnesium oxide
____________
10. Aluminum (Al+3) + Fluorine (F-1)  aluminum fluoride
____________
11. Potassium (K+1) + Iodine (I-1)  potassium iodide
____________
12. Sodium (Na+1) + Nitrogen (N-3)  sodium nitride
____________
13. Potassium (K+1) + Bromine (Br-1)  potassium bromide
____________
14. Lithium (Li+1) + Fluorine (F-1)  lithium fluoride
____________
15. Magnesium (Mg+2) + Nitrogen (N-3)  magnesium nitride
____________
1. B+3 + Cl-1
2. Mg+2 + Cl-1
3. Na+1 + Cl-1
4. Ca+2 + S-2
5. Al+3 + N-3
6. Ca+2 + Br-1
7. K+1 + Cl-1
8. B+3 + O-2
9. Mg+2 + O-2
10. Al+3 + F-1
11. K+1 + I-1
12. Na+1 + N-3
13. K+1 + Br-1
15. Mg+2 + N-3
14. Li+1 + F-1
Key
1. Boron (B+3) + Chlorine (Cl-1)  boron chloride
_____BCl3 _____
2. Magnesium (Mg+2) + Chlorine (Cl-1)  magnesium chloride ____ MgCl2 _____
3. Sodium (Na+1) + Chlorine (Cl-1)  sodium chloride
_____NaCl _____
4. Calcium (Ca+2) + Sulfur (S-2)  calcium sulfide
_____CaS _____
5. Aluminum (Al+3) + Nitrogen (N-3)  aluminum nitride
_____AlN ______
6. Calcium (Ca+2) + Bromine (Br-1)  calcium bromide
____CaBr2 ______
7. Potassium (K+1) + Chlorine (Cl-1)  potassium chloride
_____KCl _______
8. Boron (B+3) + Oxygen (O-2)  boron oxide
____ B2O3 _______
9. Magnesium (Mg+2) + Oxygen (O-2)  magnesium oxide
_____MgO ______
10. Aluminum (Al+3) + Fluorine (F-1)  aluminum fluoride
_____AlF3 ______
11. Potassium (K+1) + Iodine (I-1)  potassium iodide
_____ KI_______
12. Sodium (Na+1) + Nitrogen (N-3)  sodium nitride
____Na3N ______
13. Potassium (K+1) + Bromine (Br-1)  potassium bromide
_____KBr _______
14. Lithium (Li+1) + Fluorine (F-1)  lithium fluoride
_____ LiF ________
15. Magnesium (Mg+2) + Nitrogen (N-3)  magnesium nitride
____Mg3N2 ______
Notes to Teacher
• If you want to have the students cut out the ions themselves, they will
need two copies of the ion sheet to complete the assignment.
• In order to save time you may want to try having the students cut out
the ions as homework.
• If you are a paper-saving type, you can make class sets of ions (just
one sheet per student since they can reuse ions) and not have the
students actually paste them on the answer sheets. Instead, you can:
– Have a class set of ions premade and laminated. Put them in
ziploc bags to keep them separate. You can also laminate a class
set of answer sheets if you want the students to have a reusable
place to work out their solutions.
– Print the ion pages on magnetic paper and cover it with adhesive
laminate prior to cutting out the pieces. Then place these sets in
ziploc bags. Students can rearrange the ions on whiteboards or
cheap cookie tins (which can be purchased at most dollar stores)
before writing down their answers.