The discharge tube contains only neon gas. Electrons in the gas are excited to higher energy states when they collide with free electrons flowing from one end of the tube to the other. Light is emitted when those electrons shift back to their original states. Only a few states are available, hence only a few changes in state, each indicated by a different color of light, are permitted. Affinities of atoms and the periodic table.
Different atoms have different numbers of electrons, protons and neutrons.
The number of protons in an atom gives its atomic number . Atoms of some elements, such as helium or argon, have completely filled outer shells. The periodic table of the elements shows atomic numbers and how shells are filled with electrons for all the elements. Consequently, it is easy for those atoms to give up that one electron. For example, NaCl, the molecule with a sodium and a chlorine atom, is otherwise known as table salt.