THE PERIODIC TABLE
DOE-HDBK-1015/1-93
Fundamentals of Chemistry
CH-01
Rev. 0
Page 14
A table in which elements with similar chemical properties are grouped together is called a
periodic 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. Each
horizontal row is called a period. Note that some periods are longer than others. Elements
with similar chemical properties appear in vertical columns called groups. Each group is
designated by a Roman numeral and a capital letter, except the one on the extreme right-hand
side, Group 0 (the inert gases). At the bottom of the periodic table are two long rows of
elements identified as the lanthanide series and the actinide series. They are separated from
the table primarily to keep it from becoming too wide. Also, the elements within each of
these two series show similar chemical properties.
The number directly below each element is its atomic number, and the number above each
element is its atomic weight. In several cases the atomic weights are in parentheses. This
indicates that these elements have no stable isotopes; that is, they are radioactive. The value
enclosed in parentheses and used for the atomic weight is the atomic mass number of the most
stable known isotope, as indicated by the longest half-life.
