Why do oxidation numbers relate to valence electrons?

1 Answer
Oct 15, 2016

Because, formally, oxidation numbers relate to the loss or gain of valence electrons.


Let us take a simple oxidation reaction:

#C(s) + O_2(g) rarr CO_2(g)#

Now the oxidation numbers of the elements, dioxygen and carbon, are quite properly regarded as #"ZERO"#. They have neither donated nor accepted electrons. But in the reaction, the dioxygen oxidant is conceived to have accepted 4 electrons, and the carbon reductant is conceived to have donated 4 electrons. This is quite clear from the oxidation numbers of the elements in #CO_2#, which feature #C(+IV)# and #O(-II)# as formal oxidation states.

The same redox transfer can also be invoked for dihydrogen combustion:

#H_2(g) +1/2O_2(g) rarr H_2O(g)#

How do electrons transfer here?