1. Writing Formulas: - Ionic (Simple and Multivalent) and Covalent - a review - Polyatomic (new)

2. Writing Formulas
Remember: when writing any formula, the trick is to always balance the positive charges with the negative charges...
Cation charges + Anion Charges = 0 (neutral)

3. Writing Ionic Formulas for binary, single-valence compounds
Ionic formulas represent the ratio of Positive:Negative ions in a compound
For example: Write the formula for Magnesium Phosphide
Write the charge of each element
magnesium is Mg2+ phosphorous is P3–
Use the “Criss-Cross” Method to determine the number of each ion in the compound
Mg2+ ions P3– ions
Double check that the positive and negative charges are balanced: (2+ x 3 ions) = 6 positive : (3- x 2 ions) = 6 negative BALANCED!
Write the formula: Mg3P2
NOTE: The ratio must always be in the most reduced form and you never write a
subscript of “1” 3 2

4. What is the formula for silver sulphide?
Write the charge of each element
silver is Ag+ sulphur is S2–
Use the “Criss-Cross” Method to determine the number of each ion in the compound
Ag+ ions S2– ions
Double check that the positive and negative charges are balanced: (1+ x 2 ions) = 2 positive : (2- x 1 ion) = 2 negative BALANCED!
Write the formula: Ag2S
(Remember you don’t write the “1” subscript) 2 1

5. Writing Ionic Formulas for binary, multivalent compounds
The name of the formula will tell you which metal (positive) ion was used
For example: manganese (III) sulphide
Once you know which metal ion charge to use, follow the same rules as naming single-valence compounds...
The Mn3+ ion was used

6. What is the formula for titanium (IV) fluoride?
Write the charge of each element
titanium is Ti4+ fluorine is F–
Use the “Criss-Cross” Method to determine the number of each ion in the compound
Ti4+ ions F– ions
Double check that the positive and negative charges are balanced: (4+ x 1 ion) = 4 positive : (1- x 4 ions) = 4 negative BALANCED!
Write the formula: TiF4 (remember you don’t include the 1 subscript) 1 4

7. Writing Ionic Formulas for Polyatomic compounds
You must first identify the compound as polyatomic from the name
Hints:
The metal name starts with “ammonium”
The non-metal name doesn’t end in “-ide”
Example: phosphate
The non-metal name ends in “-ide” but the name doesn’t match an element of the periodic table
Example: cyanide (CN-) or hydroxide (OH-)

8. Writing Ionic Formulas for Polyatomic compounds
Identify the compound as polyatomic
Iron (II) phosphate
Write the charges of each ion – put brackets around the polyatomic and keep the charge outside the brackets
Iron (II) is Fe2+ phosphate is [PO4]3-
Use the “Criss-Cross” Method to determine the number of each ion in the compound
Fe2+ ions [PO4]3- ions
Double check that the positive and negative charges are balanced: (2+ x 3 ions) = 6 positive : (3- x 2 ions) = 6 negative BALANCED!
Write the formula: Fe3(PO4)2 3 2

9. X Write the formula: Fe3(PO4)2
If you forget the brackets it would be: Fe3PO42
Note: brackets aren’t needed if the subscript of the polyatomic ion is 1
Example : NaHCO3 – only 1 HCO3 ion, so no need to put
brackets with a subscript of 1 X

10. Writing Formulas for Covalent Molecules
The name of a binary covalent molecule will tell you what the formula is...
For Example:
Sulphur hexafluoride
SF6
Symbol S
Symbol F
Subscript 6

11. Let’s make a formula poster...
Rules for Writing Formulas
Is there a metal?
Yes = Ionic
Binary,
Single-Valence Formulas
Binary, Multivalent Formulas
Polyatomic Formulas
No = Covalent
Covalent Formulas
Continue with the rules for writing formulas for branch and
give an example for each...
If you finish early... Work on the review assignment – Due tomorrow