This is why nuclear energy and nuclear bombs are so strong compared to conventional chemical sources. The amount of energy that a single person uses in a lifetime is about equal to the mass of a grain of sand. The nucleus was originally discoverd by Rutherford. Electrons give an atom its size but the nucleus gives an atom its mass The defining characteristic of an element is the number of protons in its nucleus. This is called its atomic number.
Some elements have nuclei with differing numbers of neutrons.
These are called isotopes . The total number of neutrons and protons in a nucleus is called its mass number . This number is used to distinguish between different isotopes of the same element. Strong force an attractive force that overcomes the electrical repulsion between protons and holds the nucleus together. The mass of the nucleus is slightly less than the sum of the masses of the protons and neutrons individually because the missing mass is converted into binding energy.