How to find oxidation number

Asked by wiki @ 30/10/2020 in Chemistry viewed by 343 People

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

Answered by wiki @ 30/10/2020

You assign oxidation numbers to the elements in a compound by using the Rules for Oxidation Numbers.

Explanation:

The oxidation number of a free element is always 0.

The oxidation number of a monatomic ion equals the charge of the ion.

The oxidation number of $H$ $"H"$ is +1, but it is -1 in when combined with less electronegative elements.

The oxidation number of $O$ $"O"$ in compounds is usually -2, but it is -1 in peroxides.

The oxidation number of a Group 1 element in a compound is +1.

The oxidation number of a Group 2 element in a compound is +2.

The oxidation number of a Group 17 element in a binary compound is -1.

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

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 $Cr$ $"Cr"$ in ${CrCl}_{3}$ $"CrCl"_3$ ?

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 $Cr {-1 Cl}_{3}$ $"Cr"stackrelcolor(blue)("-1")("Cl")_3$

$m m m m m m l l -3 m m$ $color(white)(mmmmmmll)stackrelcolor(blue)("-3"color(white)(mm))$ .

Rule 8 states the numbers along the bottom must add up to zero. So the number under $Cr$ $"Cr"$ must be +3.

This gives $Cr {-1 Cl}_{3}$ $"Cr"stackrelcolor(blue)("-1")("Cl")_3$

$m m m m m +3 l l -3 m m$ $color(white)(mmmmm)stackrelcolor(blue)("+3"color(white)(ll)"-3"color(white)(mm))$ .

There is only one $Cr$ $"Cr"$ atom, so its oxidation number is +3.

This gives $+ 3 Cr {-1 Cl}_{3}$ $stackrelcolor(blue)(+3)("Cr")stackrelcolor(blue)("-1")("Cl")_3$

$m m m m m +3 l l -3 m m$ $color(white)(mmmmm)stackrelcolor(blue)("+3"color(white)(ll)"-3"color(white)(mm))$ .

The oxidation number of $Cr$ $"Cr"$ in ${CrCl}_{3}$ $"CrCl"_3$ is +3.

Here is a chart showing the oxidation numbers of the atoms in some common elements and compounds.

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How to find oxidation number

Explanation:

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