How do you calculate the oxidation number of an element in a compound?

How do you calculate the oxidation number of an element in a compound?

You assign oxidation numbers to the elements in a compound by using the

Rules for Oxidation Numbers.

Explanation:

1. The oxidation number of a free element is always 0.
2. The

   oxidation number of a monatomic ion equals the charge of the

   ion.

3. The oxidation number of %H% is +1, but it

   is -1 in when combined with less electronegative elements.

4. The

   oxidation number of %O% in compounds is usually -2,

   but it is -1 in peroxides.

5. The oxidation number of a Group 1

   element in a compound is +1.

6. The oxidation number of a Group 2

   element in a compound is +2.

7. The oxidation number of a Group 17

   element in a binary compound is -1.

8. The sum of the oxidation

   numbers of all of the atoms in a neutral compound is 0.

9. The sum of

   the oxidation numbers in a polyatomic ion is equal to the charge of the

   ion.

EXAMPLE:What is the oxidation number of

%Cr% in %CrCl_3%

?Solution:We use what rules we can to determine the

oxidation numbers.Rule 7 states that the oxidation number

of Cl is -1.We write the oxidation number of the element above its symbol

and the total for 3 Cl atoms below the symbol.This gives

%Crstackrelcolor(blue)(-1)(Cl)_3%
%color(white)(mmmmmmll)stackrelcolor(blue)(-3color(white)(mm))%.Rule

8 states the numbers along the bottom must add up to zero. So the

number under %Cr% must be +3.This gives

%Crstackrelcolor(blue)(-1)(Cl)_3%
%color(white)(mmmmm)stackrelcolor(blue)(+3color(white)(ll)-3color(white)(mm))%.There

is only one %Cr% atom, so its oxidation number is

+3.This gives

%stackrelcolor(blue)(+3)(Cr)stackrelcolor(blue)(-1)(Cl)_3%
%color(white)(mmmmm)stackrelcolor(blue)(+3color(white)(ll)-3color(white)(mm))%.The

oxidation number of %Cr% in

%CrCl_3% is +3.Here is a chart showing the oxidation

numbers of the atoms in some common elements and

compounds.