Periodic Table
Article by: Kaner, Richard B. , Department of Chemistry and Biochemistry, University of Los Angeles, Los Angeles, California.
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A list of chemical elements arranged along horizontal rows in increasing atomic
number. It is organized such that the vertical columns consist of elements with remark-
ably similar properties (see illustration). The first column, known as the alkali metals
(albeit with hydrogen, a nonmetal on top), contains elements with just one outer (valence)
electron. The last column has completely filled valence orbitals leading to chemically inert
elements called the noble gases. The position of elements in the periodic table provides a
powerful method of classifying not only the physical properties of elements but also their
expected properties in molecules and solids.
The periodic table dates back to around 1870 when the Russian chemist D. Mendeleev
used the similarities in chemical reactivity attributed to different elements to group them
according to increasing atomic mass. However, this left blank spaces that Mendeleev
boldly predicted would be filled by elements that were then undiscovered. Not only did
other scientists discover missing elements including scandium, gallium, and germanium,
but remarkably these elements possessed properties similar to Mendeleev's predictions.
Although the layout of the periodic table has changed over time with the addition of new
elements, the essential information remains comparable to the original periodic table.
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Content
• Groups
• Periods
• Elements
• Other properties
Key Concepts
• Originally developed by the Russian chemist D. Mendeleev, the periodic table
lists every known chemical element in horizontal rows ("periods") by increasing
atomic number.
• Each cell in the periodic table contains a symbol representing the element and its
atomic number.
• Each column of the periodic table contains a family (group) of chemicals with
the same number of valence electrons and, therefore, similar chemical properties.
• The left side of the periodic table includes the alkali (group 1) and alkaline earth
(group 2) metals, followed by the transition metals in the center.
• The right side of the periodic table contains the nonmetals (except hydrogen),
including halogens (group 17) and noble gases (group 18).
