Political Science homework help. You assign to the elements in a compound by using the Rules for Oxidation Numbers.

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”## is +1, but it is -1 in when combined with less electronegative elements.

The oxidation number of ##”O”## in 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”## 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 ##”Cr”stackrelcolor(blue)(“-1”)(“Cl”)_3##
##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”## must be +3.
This gives ##”Cr”stackrelcolor(blue)(“-1”)(“Cl”)_3##
##color(white)(mmmmm)stackrelcolor(blue)(“+3″color(white)(ll)”-3″color(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)(“+3″color(white)(ll)”-3″color(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.

Political Science homework help