The basic particles of an atom are the proton, neutron and electron. One model of an atom pictures a dense positively charged nucleus. A cloud of negative electricity surrounds it. The cloud is produced by electrons orbiting around the nucleus. The nucleus contains neutrons and positively charged protons.
A nucleus is only a tiny fraction of an atom. Yet it contains almost the total mass of an atom. The mass of an electronic is almost nothing compared to the mass of proton or a neutron. The mass of one proton or one neutron equals the mass of 1837 electrons.
Atoms are assigned a mass number based on a scale of atomic mass units. Atomic mass unit is abbreviated as a.m.u For example, a helium nucleus has a mass of 4 a.m.u. Sodium has a mass of 23 a.m.u. Sulphur has a mass of 32 a.m.u. The mass is about equal to the Sim of the atom’s protons and neutrons.
The entire universe is held together by forces. The force of gravity hold the parts of the solar system together. Atoms and molecules are held together by the ionic and magnetic forces of charged particles. Nuclear forces hold the parts of an atomic nucleus together.
Nuclear forces hold protons and neutrons together in a nucleus. However, they are not like the force of gravity of the forces between charged electrical particles. Nuclear forces exist only on distances shorter than the size of an atomic nucleus (10*-2 or less). Thus, at distances of 10*-2 our less, protons are held together by nuclear forces. At larger distances, the positive charges of protons cause then to repel reach other. Nuclear forces also cause protons and neutrons to bond together within a nucleus.