From the chapter "Structure of an Atom," students get to know about the atom. The particle which can't be further divided, what is it made up of. The great scientist J.J. Thompson had discovered the electrons, and scientist E. Goldstein had discovered protons. The scientist J.J. Thompson said that the electrons are closely packed in the positive sphere. The great scientist Rutherford had discovered the nucleus of an atom with the help of the famous experiment, i.e. scattering experiment using the alpha particle. By this experiment, many theories emerged. Let's see them one by one.
With the help of the alpha particle scattering experiment, it was clear that the nucleus is placed in the centre, and electrons revolve somewhere outside the nucleus. This became the hot discussion of that time since it was unknown how the nucleus is stabilised inside. After that, Neils Bohr came up with his theory and demonstrated the model of an atomic particle. He explained that the electrons are placed at discrete energy levels around the nucleus and revolve at their positions, and if shells are filled(complete), then atoms will be stable and less reactive. James Chadwick discovered the existence of neutrons. These are present in the nucleus of an atom along with the protons. Thus, by now, the sub-atomic particles were three in number- electron, neutron, and proton. And they defined charge also, i.e. negative charge is the charge of an electron and neutral is of the neutron, and positive charge is the charge of a proton. So the atomic number (Z) of an element is defined as the number of protons contained in an atom of the element.