In this and in the few next paragraphs we will discuss the structure and the characteristics of the atomic nucleus. In these paragraphs you can read about such phenomena like the radioactivity, the nuclear transformation, the nuclear fission, and finally about the atomic bomb, and the nuclear reactor. Those are very interesting issues and they are also important aspects of nowaday living.
But let's start from the beginning. 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 like 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 is an constituent of the nucleus. It is the proton. It emerged that that particle is 1836 times heavier than the electron and has the same amount of charge but of the opposite (positive) sign. So the hydrogen atom could have been pictorially described as one proton and one electron rotating around it.
But it was soon discovered that except for the proton there is also a second constituent of the nucleus. In 1930 the two German scientists Walter Bothe and H. Becker discovered a new phenomenon. They bombarded a beryllium plate with the alpha particles (the helium nuclei). They noticed the emission of a very penetrating radiation. The radiation as it was soon discovered was not deflected in a magnetic field and could penetrate even a several centimetres thick layer of lead. The Englishman James Chadwick noticed that the radiation consisted of particles holding no charge (neutral). Their mass was very similar with the mass of the protons. Chadwick demonstrated that the particles he had discovered were the second constituent of the nucleus. They are called the neutrons.
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. As it was mentioned the nuclei of various elements consist of various numbers of protons. And what about the neutrons? Well, the nuclei of one element (of one Z number) can consist of various numbers of neutrons! You can read about that on the next page.
REMEMBER:
The atomic nucleus consists of protons and neutrons.
The proton has a positive charge and the neutron has no charge.
The proton is 1836 times heavier than the electron.
The atomic number Z represents the number of protons in the nucleus, and the mass number A represents the mass of the nucleus.
Proton was discovered by Rutheford, and neutron by Chadvick.
