The proton, which has a positive electrical charge, and the neutron, which has no charge, are concentrated in an area at the center of an atom, called the nucleus . These two particles have essentially the same mass. Each element has a unique number of protons, and every atom of that element has the same number. The nucleus is held together only by powerful counterbalancing forces, called nuclear forces, which hold the nucleus together, like a glue. We'll come back to this later.
Atomic weight is another feature describing atoms and elements.
The atomic weight is the number of protons and neutrons in the nucleus. The number of protons is all atoms of a particular element is constant, however, the number of neutrons may vary. Atoms of an element that have different atomic weights are called isotopes . They vary only in the number of neutrons. Atoms of an element may vary in the number of electrons they possess.