As you probably remember in 1911 Ernest Rutheford created a new, pictorial model of the atom. It was to consist of a small but heavy atomic nucleus having a positive charge, and of the electrons rotating around that nucleus. The complete charge of the electrons is the same as of the nucleus. So the whole atom is neutral. But another question arises - is the nucleus homogeneous or does it consist of some particles? As soon as in 1919 Ernest Rutheford discovered the particle that was a constituent of the nucleus. It was the proton. It emerged that that particle was 1836 times heavier than the electron and had the same amount of charge but of the opposite (positive) sign. The mass of the proton is equal to 1.6726*10^-27 kg. So the hydrogen atom could have been pictorially described as one proton and one electron rotating around it.

Soon it was discovered that except for the protons the nuclei consisted also of another kind of particles In 1930 two German scientists Walter Bothe and H. Becker bombarded beryllium (⁹⁴Be) with the alpha particles (the nuclei of helium). They observed the emission of a very hard radiation. The radiation was not deflected in magnetic field and could penetrate even a several centimetres thick layer of lead. The scientists assumed that it was a high energetic gamma radiation. But soon they discovered that its energy was much higher than any gamma radiation could have. Irene Joliot Curie and Frederic Joliot Curie noticed also that the radiation knocked protons out the paraffin slug.

So the atomic nucleus consists of two kinds of particles: the neutrons, and the protons. The common name to them is the nucleons. The nuclei of various elements consist of various numbers of protons. In the periodic table the number of protons is represented with the atomic number Z. For example the hydrogen nucleus includes one proton, the oxygen nucleus - eight, and the silver nucleus - forty-seven. The number of the electrons rotating around the nucleus is the same as the number of the protons in that nucleus. And so around the hydrogen nucleus there is one electron rotating, around the oxygen nucleus - eight, and around the silver nucleus - forty-seven. The mass of the nucleus consist of the masses of all the protons and neutrons summed together. In the periodic table the mass of the nucleus is represented with the mass number A. So if a nucleus consists of 5 protons and 5 neutrons then the atomic number is equal to 5, and the mass number is equal to 10. And vice versa an atom of Z=7 and A=16 consists of 7 protons, 9 neutrons, and 7 electrons.