Writing Formulas

Oxidation Numbers & Chemical Formulas

Mrs. Wolfe

Review: How do you figure out the charge of an ion?

• What is the charge of Sodium?

• +1

• Another word for Charge is OXIDATION NUMBER.

+ -

Is there a pattern of Oxidation Numbers on the Periodic Table?

• Time for a Worksheet: “Discovering Oxidation Numbers”!

+1

+2 +3 ±4 -3 -2 -1

0

Write the pattern in your notes!

There is a pattern of ions!

• METALS ARE POSITIVE!

• NONMETALS ARE NEGATIVE!

Practice Problems…

What is the oxidation number of Aluminum?

+3

What is the oxidation number of Iodine?

-1

Time for a worksheet: Oxidation Numbers

Writing Formulas

• Consider the formula BeCl2

• How many atoms of Beryllium are in that compound?

• 1• How many atoms of

Chlorine?• 2• WHY?

BeCl2

Be

2

Cl

7

Cl

7

Cl -1

BeCl2

Be +2

Cl -1 Cl -1 Be +2

Lithium & Fluorine

• What is the oxidation number for Lithium?

• +1• Fluorine?• -1• So how many atoms do you

need of each element to make a compound?

• One of each!• The formula is…

LiF

F -1 Li +1

LiF not FLi…why?

• You always write the METAL FIRST in a formula.

• Let’s try another one.

• Try to write a formula for Magnesium & Sulfur.

• MgS

S -2 Mg +2

How about Magnesium + Chlorine?

• Magnesium’s oxidation #• +2

• Chlorine’s oxidation #• -1

Cl -1 Mg +2 Cl -1

• MgCl2

Time for some practice with Formula Writing!