1. Forming Ionic Compounds Adrena McDonald Venture Alternative High School

2. 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 a couple together.

3. boron chloride BCl3 1. BCl3 Cl-1 B+3 Cl-1 Cl-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

4. magnesium chloride MgCl2 2. MgCl2 Cl-1 Mg+2 Cl-1
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

5. Now, it’s your turn.
Do the remaining problems on your own. Then view the remainder of the slides.

6. 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.
B+3 Cl-1 3 1
B Cl 3 1
BCl3

7. Magnesium (Mg2+) + Nitrogen (N-3) Magnesium nitride
Here’s another example of using the criss-cross method.
Mg+2 N-3 2 3
Mg N 2 3
Mg3N2

8. Potassium (K+1) + Sulfate (SO4-2)  potassium sulfate
This also works for polyatomic ions.
K+1 (SO4)-2 1 2
K (SO4)
Parentheses are not used unless a subscript is needed after the polyatomic ion, as you will see in the next example.
K2SO4

9. Al+3 (SO4)-2 Al (SO4) Al2(SO4)3 3 2
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.
Al+3 (SO4)-2 3 2
Al (SO4)
Al2(SO4)3

10. If the superscript charges add to zero, DO NOT criss-cross
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
B+3 + N-3  BN
(+2 and -2 = 0)
(+3 and -3 = 0)

11. Released TAKS Question
calcium (Ca+2) + chlorine (Cl-1)  calcium chloride
The chemical formula for calcium chloride is ---
A) Ca2Cl
B) CaCl
C) CaCl2
D) Ca2Cl3
Ca+2 Cl-1 2 1
Ca Cl 2 1
CaCl2

12. Released TAKS Question
According to this information, what is the chemical formula for aluminum sulfate?
A) AlSO4
B) Al2(SO4)3
C) Al3(SO4)2
D) Al6SO4
Al3+ SO42-
Al+3 (SO4)-2 3 2
Al (SO4)
Al2(SO4)3