Oxidation number is conceived to be the charge associated with a given atom, when it donates or accepts electrons according to predetermined rules...given here for your chemical pleasure....
1. The oxidation number of a free element is always 0.
2. The oxidation number of a mono-atomic ion is equal to the charge of the ion.
3. For a given bond, X-Y, the bond is split to give X+ and Y−, where Y is more electronegative than X.
4. The oxidation number of H is +1, but it is -1 in when combined with less electronegative elements.
5. The oxidation number of O in its compounds is usually -2, but it is -1 in peroxides.
6. The oxidation number of a Group 1 element in a compound is +1.
7. The oxidation number of a Group 2 element in a compound is +2.
8. The oxidation number of a Group 17 element in a binary compound is -1.
9. The sum of the oxidation numbers of all of the atoms in a neutral compound is 0.
10. The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.
And thus for elemental sodium we gots Na(0).. Sodium is a good reductant, and for most of its reaction chemistry we get Na+≡Na(I+)... In the given scenario, the metal has NEITHER accepted nor donated its single valence electron...
