Electronegativity is an atoms ability to attract electrons to itself in a chemical bond. Since sodium and potassium ions are not covalently bonded to anything, the concept of electronegativity is not relevant here. Both ions will have full +1 charges.
The way the sodium-potassium pump works is that it pumps 3 sodium ions out of the cell, and 2 potassium ions in, leading to a sodium gradient, a potassium gradient, and an electrical gradient (since the net result is pumping one positive charge out of the cell). These three gradients allow the neuron to very precisely control its electrochemical state and perform the tasks needed to generate action potentials:
1) The sodium gradient allows cells to depolarize by opening sodium channels.
2) The potassium gradient allows cells to hyperpolarize opening potassium channels.
3) The electrical gradient creates a membrane potential that can be sensed by voltage-gated channels.
All three of these concepts (depolarization, hyperpolarization, and voltage-sensing) are crucial to action potentials.