1. What are oxidation numbers?
Lesson 26
What are oxidation numbers?

2. Atoms of metals link up with atoms of non-metals. They form compounds.
When a compound forms, the metal lends outer ring electrons to the non-metal.

3. The non-metal borrows electrons.
How many are lent and borrowed? It depends on the atom and the compound being made.

4. The number of electrons an atom can lend or borrow is called its oxidation number.
An oxidation number has a plus sign (+) or a minus sign (-) next to the atoms it describes.
Al+3, Mg+2, Br-1, Se-2

5. The sign (+ or -) tells us whether the atom lends or borrows electrons.
+ means they lend (they have extra)
- Means they borrow (they don’t have enough)
The number tells us how many

6. Sodium has an oxidation number of +1 (Na+1) This means that sodium can lend 1 electron.
Oxygen has an oxidation number of -2 (O-2). Oxygen can borrow 2 electrons.

7. Remember:
Metals lend, they have + oxidation numbers.
Non-metals borrow, they have – oxidation numbers.
Non-metals like complete outer shells, so they borrow enough to make it complete.

8. Many elements have more than one oxidation number
Many elements have more than one oxidation number. In fact, some elements have both plus and minus oxidation numbers!
Sometimes these elements borrow electrons, and sometimes they lend.

9. You can use oxidation numbers to figure out the formula for a compound.
You don’t need to worry about the + or – signs.
Just crisscross the numbers.

10. Step 1: Write down the elements and their oxidation numbers
H O-2

11. Step 2: Write the symbols again, below where you already have them written.
H+1 O-2
H O

12. Step 3: Crisscross the numbers to the bottom, leave out the signs.
H O-2
H O1

13. Step 4: Move them all together, and leave out any 1s
H2O
If there is no number, it is understood that there is only 1 atom.

14. Let’s try finding the formula for table salt
Let’s try finding the formula for table salt. It is made of sodium, (Na+1) and chlorine (Cl-1)
Na Cl-1
Na Cl 1 1
NaCl