In 1869, the Russian chemist Dmitri Mendeleyev published his great systematization called the periodic table. He arranged all known chemical elements in order of their atomic masses and found that similar physical and chemical properties recurred every 7 elements for the lighter elements and every 17 elements for the heavier ones. (The inert gases had not been discovered at that time; the correct values for similar properties are 8 and 18.) The periodic table is based on atomic masses and similar properties. In each row, the atomic masses increase toward the right. Each column contains a group of elements with similar chemical behavior.
In the modern periodic table, each box contains four data, as shown in Figure 1. Besides the element name and symbol, the atomic mass is at the bottom, and the atomic number is at the top. The elements are arranged in order of increasing atomic number in horizontal rows called periods.
Figure 1. Facts given for each element.
In the preceding section, you reviewed the historical determination of atomic masses. Notice that the elements also seem to be arranged in order of increasing atomic mass. But several exceptions exist. Compare the atomic mass of tellurium (Te) to iodine (I). (See Figure 2.)
Figure 2. Comparing tellurium and iodine.
Consider the proper placement of tellurium and iodine in the periodic table, as shown in Figure 2. Te has the heavier atomic mass. The chemical properties of tellurium are like those of selenium because both are semimetallic elements that form compounds like those of sulfur. Iodine resembles bromine because these elements are nonmetallic halogens that form compounds like those of chlorine. Therefore, the order in the table cannot be based solely on atomic mass.
The atomic number, which appears above each element symbol, represents the meaningful order in the periodic table. When an element is referred to by an integer, this number means the atomic number, not the atomic mass. Thus, element 27 is cobalt (whose atomic number is 27), not aluminum (whose atomic mass is 27).
The periodic table displays the pattern of properties of the elements. The lightest are at the top of the chart; the atomic masses increase toward the bottom of the chart. The elements to the upper right, above a diagonal line from aluminum (13) to polonium (84), are nonmetals, about half of which exist as gases under normal laboratory conditions. All the elements in the middle and left of the table are metals, except gaseous hydrogen (1). Most of the metals are shiny, deformable solids, but mercury has such a low melting point that it is a liquid at room temperature. All the metals have high conductivities for heat and electricity. Many simple chemical compounds are formed from a metal reacting with a nonmetal.
In the periodic table, shown on the following pages, elements in columns have similar properties, and elements so related (like sulfur, selenium, and tellurium) are members of the same group or family and are congeners of one another.
This information was obtained from the International Union of Pure and Applied Chemistry (IUPAC) Web site: www.IUPAC.org.