Single electron reduction of ketone by Na or Zn
Let the carbonyl compound under consideration be acetone.

Case I: Hydrogen Abstraction



The species formed are NaH and an allyl-type radical. The resonating system includes an electronegative oxygen atom, which also has an incomplete octet in one of the canonical forms.

In essence, products are not really stable.

Case II: Attack on Carbonyl


The species formed is a radical anion, which are relatively stable as the filled orbital of the oxygen relieves some electron deficiency of the radical. This effect is stronger than the one offered the pi-bond in Case I.