What is the sum of all force in an object when not acceleration

Zero  

Explanation:

Newton's first law states that an object at rest tends to remain at rest, and an object in motion tends to remain in motion, unless it is acted on by a net force.

A net force is the sum of all the forces acting on the object.

If the object is not being accelerated, the net force is zero.

a electronegativity decreases beacuse the size of nucleus is bigger.