06.Periodicity of Behavior;
       Sodium Through Argon		   Previous PageNext Page
       Oxoidation States and Oxidation Numbers

The last rule follows from the fact that oxidation numbers reflect the pushing and pulling of electrons between atoms, and that the total number of electrons must remain constant during any chemical process, such as the building of a molecule from its component atoms.

With these three short cuts, you can see quickly how the ON of chlorine varies in its acids and oxyacids:

Molecule ON Cl
ON H
 Total ON O Sum
HCl
-1
+1
-
0
Cl2
0
-
-
0
HClO
+1
+1
-2
0
HClO2
+3
+1
2(-2)
0
HClO3
+5
+1
3(-2)
0
HClO4
+7
+1
4(-2)
0

 

 

The physical meaning of these oxidation numbers is that, in the series from HCl to HClO4, the chlorine atom loses more and more of its grasp over the electrons that it shares with other atoms.

In HCl it easily dominates over the weakly electronegative proton, but in HClO4 it does an ineffective job of competing against the stronger electron-pulling power of the oxygen atoms.

Chlorine is oxidized in going from Cl2 to HClO4, even though the change represents only a shifting away of electrons and not their outright removal.

Whenever an atom is oxidized, its ON increases, and whenever it is reduced, its ON decreases.

Chemists commonly refer to the oxidation state of an atom, meaning its oxidation number in a specified compound.
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