A basic explanation of these processes can be accomplished without using quantum theory. A short reference to Bohr's model of the atom will suffice. According to this model, an atom consists of a heavy nucleus and a number of electrons arranged on well defined shells around this nucleus. With increasing distance from the nucleus, these shells are designated with the letters K, L, M, N, O, P, Q, etc. All nuclei, except that of regular hydrogen, contain besides the positively charged protons an almost equal number of charge free neutrons. The number of protons in the nucleus corresponds to the element number of the material. In an electrically neutral atom, the number of protons and the number of electrons are equal. The closer the electrons are to the nucleus, the tighter they are bound to the nucleus by its' electric field, or (in other words) the more energy is needed to push them out of their place on the shell.