 The modern Periodic Table depicted in Figure 1 is based on Mendeleev’s table, but now includes 92 naturally occurring elements and some dozen artificial elements (see Scerri 2006). The lightest element, hydrogen, is difficult to place, but is generally placed at the top of the first group. Next comes helium, the lightest of the noble gases, which were not discovered until the end of the 19th century. Then the second period begins with lithium, the first of the group 1 (alkali metal) elements. As we cross the second period, successively heavier elements are first members of other groups until we reach neon, which is a noble gas like helium. Then with the next heaviest element sodium we return to the group 1 alkali metals and begin the third period, and so on.