THE PERIODIC TABLEDOE-HDBK-1015/1-93Fundamentals of ChemistryCH-01Rev. 0Page 14A table in which elements with similar chemical properties are grouped together is called aperiodic table. One of the most common versions is shown in Figure 3. In this table,elements are arranged in order of increasing atomic number in succeeding rows. Eachhorizontal row is called aperiod. Note that some periods are longer than others. Elementswith similar chemical properties appear in vertical columns called groups. Each group isdesignated by a Roman numeral and a capital letter, except the one on the extreme right-handside, Group 0 (the inert gases). At the bottom of the periodic table are two long rows ofelements identified as the lanthanide series and the actinide series. They are separated fromthe table primarily to keep it from becoming too wide. Also, the elements within each ofthese two series show similar chemical properties.The number directly below each element is its atomic number, and the number above eachelement is its atomic weight. In several cases the atomic weights are in parentheses. Thisindicates that these elements have no stable isotopes; that is, they are radioactive. The valueenclosed in parentheses and used for the atomic weight is the atomic mass number of the moststable known isotope, as indicated by the longest half-life.
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