Now, the atomic weight of an element is a function of how many particles it contains, primarily protons and neutrons, since the mass of the electron is relatively negligible. Look at hydrogen. Its atomic weight is about one. Helium, with two protons and two neutrons, has an atomic weight of about four. A century and a half ago, a Russian chemist named Dmitri Mendeleev demonstrated that if you write down the atomic weights of the chemical elements, you will see that similar chemical properties occur in a periodic fashion. That is, start with hydrogen, atomic weight one. It has unique chemical properties. Those chemical properties are very similar to those of lithium, atomic weight 3, and also to sodium, atomic weight 11, and potassium, atomic weight 19. Notice the interval of 8 between lithium, sodium and potassium. This is highly suggestive of a pattern correlating atomic weight with chemical properties, but of course the pattern starts out with an interval of 2, not eight. Similar imperfect patterns were apparent, in part because Mendeleev had to work with atomic weight rather than atomic number. Mendeleev got quite dour and scruffy-looking before he figured it all out. His work eventually led to the development of the periodic table of the elements.