Atomic Number is equal to the number of protons in the nucleus. It defines the element. The Mass Number or Atomic Weight is the weighted average of the number of protons and neutrons in the atoms of an element. Isotopes Isotopes are atoms of the same element with a different number of neutrons. This is why the Mass Number isn't always 2x the atomic number.
Isotopes can be stable or unstable.
They decay and lose some of their nucleus and become different elements. The electrons to stay as close as possible to the nucleus. As far away from each other as possible. There is an finite number of electrons which can occupy each energy level. Whether an atom donates and electron.