Answer
Sodium has a positive charge and Oxygen has a negative charge because of having only one bond. Both the charges cancel each other to give an overall charge of zero.
Work Step by Step
Formal Charge (FC) = [number of valence electrons] - [number of non-bonding electrons] - 1/2 [shared electrons]
FC (sodium) = 1-0-0 = +1
FC (oxygen) = 6-6-1 = -1
FC (hydrogen)= 1 -0-1 = 0
