Chapter 42 will discuss nuclear physics in more detail, but it will be helpful to give a brief overview. The relative masses of many of the elements were known from chemistry experiments by the mid-19th century. By arranging the elements in order of ascending mass, and noting recurring regularities in their chemical properties, the Russian chemist Dmitri Mendeleev first proposed the periodic table of the elements in 1869. But what did it mean to say that hydrogen was atomic number 1, helium number 2, lithium number 3, and so on?
It soon became known that hydrogen atoms can be only singly ionized, producing . A doubly ionized is never observed. Helium, by contrast, can be both singly and doubly ionized, creating and , but is not observed. It seemed fairly clear, after the work of Thomson and Millikan, that a hydrogen atom contains only one electron and one unit of positive charge, helium has two electrons and two units of positive charge, and so on. Thus the atomic number of an element, which is always an integer, describes the number of electrons (of a neutral atom) and the number of units of positive charge in the nucleus. The atomic number is represented by Z, so hydrogen is , helium , and lithium . Elements are listed in the periodic table by their atomic number.
Rutherford’s discovery of the nucleus soon led to the recognition that the positive charge is associated with a positive subatomic particle called the proton. The proton’s charge is , equal in magnitude but opposite in sign to the electron’s charge. Further, because nearly all the atomic mass is associated with the nucleus, the proton is much more massive than the electron. According to Rutherford’s nuclear model, atoms with atomic number Z consist of Z negative electrons, with net charge , orbiting a massive nucleus that contains protons and has net charge . The Rutherford atom went a long way toward explaining the periodic table.
But there was a problem. Helium, with atomic number 2, has twice as many electrons as hydrogen. Lithium, , has three electrons. But it was known from chemistry measurements that helium is four times as massive as hydrogen and lithium is seven times as massive. If a nucleus contains Z protons to balance the Z orbiting electrons, and if nearly all the atomic mass is contained in the nucleus, then helium should be simply twice as massive as hydrogen and lithium three times as massive.