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.