We will answer these questions soon, but first we need to look into the nature of the nucleus and quantum mechanics.

Protons are positive and electrically repel one another. A nucleus would blow apart if it weren't "glued" together by the gluon particles which affect every part of the nucleus. This is called the residual strong force.

Think of a nucleus as a tightly coiled spring which is the electrical repulsion, held in place by a very big rope which is the residual strong force. Even though there is a lot of stored-up energy in the spring, it can't release the energy because the rope is too strong.