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periodic table
Periodic Table courtesy of the US Department of Energy, Berkeley Lab

periodic table:

A table of the elements in the order of their atomic numbers, arranged in horizontal rows and vertical columns of related groups. Each row, or period, contains elements with the same number of electron shells. Each column, or period, contains elements that have similar chemical properties because their outermost electron shells contain the same number of electrons. In 1869, Dmitry Ivanovich Mendelyev published the first reasonably complete periodic table based on his discovery that the properties of elements vary periodically with increasing atomic weight. Gaps in the table corresponded to elements then unknown. Using the table, Mendelyev accurately predicted the properties of these elements even before they were discovered. The table is added to as new and heavier elements are made in laboratories. Currently, there are 115 known elements though they are not in sequence.