A nucleon is a proton or a neutron - part of the nucleus.

The kinetic energy is the energy of motion, the energy an object possessed, e.g. a nucleon, a nucleus, an atom or another particle, because of its motion.

The mass number is the total number of protons and neutrons (also total nucleon) in the nucleus.

See also Atomic Mass Unit, Atomic Mass.

Ernest Rutherford and Niels Bohr developed in the early 20th century a solar system like model of the atoms, in which electrons orbit around the nucleus (protons and neutrons) held by electromagnetic forces (protons - electrons).
The nucleus is held together by a very strong but short distance nuclear force, attracting all nucleons. While the protons positive charges try pushing it apart, is it the balance between protons and neutrons which decide over an elements stability.
In their model the energy of orbiting electrons is quantized into fixed values. Electrons in outer orbits are more loosely bound than the ones at inner orbits and affect an atom's chemical properties.
Erwin Schrodinger and Werner Heisenberg developed probability functions which assigns the electrons to cloud like spaces instead of fixed orbits.