1. Formula Construct Acitivity GOHS Chemistry
4.6 – Writing Formulas

2. How do you write formulas for binary ionic compounds given the name?
Two simple steps:
Write the symbol and charge of each ion
Balance the charges by providing subscripts
Magnesium chloride
Write the symbol and charge of each ion.
Mg Cl-1
Cl-1
Balance the charges by supplying subscripts. Subscripts tell how many of each atom is present.
MgCl2
You need a second Cl-1 to balance the charges

3. FeBr3 More examples: Iron III bromide
Write the symbol and charge of each ion. The charge of the iron is provided by the Roman Numeral.
Fe Br -1
Br -1
Br -1
FeBr3
Balance the charges by supplying subscripts. The subscripts tell how many of each ion is needed to balance the compound.
You’ll need three bromine ions to balance the one iron.

4. Al2S3 Aluminum Sulfide Write the symbol and charge of each ion.
Balance the charges by supplying subscripts.
S-2
In this case the charges do not evenly divide into each other. You must find the least common multiple. SIX
Al2S3
How many aluminums are needed to arrive at a +6 charge? 2
How many sulfurs are needed to arrive at a -6 charge? 3

5. How do you write formulas for Polyatomic Ion Compounds?
Very much like writing formulas for Binary Ionic Compounds.
Two steps:
Write the formulas/symbols of each ion.
Balance the charges by supplying subscripts.
If a subscript is needed for a polyatomic ion, it must be put in parentheses with the subscript on the outside.
Let’s do some!!

6. Al(NO3)3 Balance the charges by supplying subscripts.
Aluminum nitrate
First, you can tell from the name that there is a polyatomic ion present (nitrate). All binary ionic compounds have suffixes of –ide.
Al NO3-1
NO3-1 NO3-1
Write the formula/symbol and charge of each ion.
Balance the charges by supplying subscripts.
Since Al is +3 and NO3 is -1, you need a total of 3NO3-1 to balance one Al+3
Since you will need a subscript of 3 for NO3-1, you need to put this in parentheses with the 3 outside.
Al(NO3)3

7. Ca3(PO4)2 Pb(C2H3O2)4 Lead (IV) acetate
Write the formula/symbol and charge of each ion.
Pb C2H3O2-1
C2H3O2-1
Balance the charges by supplying subscripts. Since Pb is +4 and C2H3O2 is -1, you will need 4 C2H3O2-1 to balance 1Pb+4
Pb(C2H3O2)4
Write the formula/symbol and charge of each ion.
Calcium phosphate
Ca PO4-3
Balance the charges by supplying subscripts. Since Ca is +2 and PO4 is -3, you will need 3 Ca+2 to balance 2 PO4-3
Ca+2 PO4-3
Ca+2
Ca3(PO4)2

8. “Cross-Over Method” Al3+ O2- example: aluminum oxide
The correct formula is Al2O3
Always make sure the combined charges add up to zero
(2 × 3+) + (3 × 2–) = 0

9. Let’s practice Write the formulas: H+ Cl- Cu2+ Br- K+ O2- Co3+ CO32-
Pb2+ O2-
Pb4+ O2-
Answers
HCl
CuBr2
K2O
Co2(CO3)3
PbO
PbO2