2.3 The Periodic Table
Here (Figure 2.3) we see the Periodic Table of the Elements. Elements are ordered by increasing atomic number (the number of protons in their atom nucleus), which correlates with atomic weight and size. The first element is hydrogen (atomic number 1) and the last is oganesson (atomic number 118). Oganesson is an obscure synthetic radioactive element that has only been produced a few times and in minute amounts. In this table, elements are in numbered periods (rows) and groups (columns) based on the configuration of their electron orbitals.
The chart contains 118 elements in all, but only about 90 occur naturally. The other 20 or so (with atomic numbers 104-118 and 99-103) elements are artificial – they are synthesized in nuclear reactors and some are radioactive with very short lives. The distinction between natural and artificial elements is, however, a bit uncertain because only a few atoms have (speculatively) been identified for some of the rarest natural elements. Although the Periodic Table of the Elements has appeared in many forms, the basic relationships are the same today as they were when Dmitri Mendeleyev (1834-1907) devised the first version in 1870. Some elements were unknown and omitted from the original table during Mendeleyev’s time, but were later discovered and added. Additionally, there has been a slight rearrangement so that the arrangement of the elements mirrors the order in which electrons occupy orbitals.
Chemists classify elements into different types that have related properties (shown by different colors in the table above). Hydrogen is a special element, but the other elements in Group 1 are alkali metals. The elements in group 2 are alkaline earth metals. Those elements in Groups 4 through 12 are transition metals. Group 17 elements are the halogens, and Group 18 elements are the noble gases. Elements between the transition metals and the halogens are called nonmetals or other metals (but different versions of the chart have them divided in slightly different ways). The asterisks in the chart show where the lanthinides (also called rare earth elements) and actinides (both considered transition metals) were extracted from the main chart and put as separate rows at the bottom. If we did not extract them, the chart would be too wide to fit easily on this page. Scandium (Sc) and yttrium (Y) are closely related to the lanthinides and so are commonly classified with them.