Periodic Table

[AndrewD]

What is the periodic table?

[Freddie]

The Periodic Table is a table devised by chemists that arranges all the known chemical elements in a way that is useful to them. The elements are arranged from left to right and top to bottom in order of increasing atomic number or atomic mass.

[Ed Ancona]

Extending Freddie's answer -- if the elements are arranged so that those with similar basic properties are in vertical rows, there is a periodicity evident in the arrangement. All halogens, for instance -- flluorine, chlorine, bromine -- or the alkali elements -- lithium, sodium, potassium -- are in vertical rows. This arrangement frst proposed by the scientist, Mendeleyev, is useful in describing the atomic structure of the various elements. The table extends from the lightest elements at the top, starting with hydrogen, to the heaviest at the bottom, uranium. Or at least uranium was the heaviest until nuclear scientists began to create the "transuranium" elements in the "atomic piles," iincluding plutonium and many named for scientists, such as Einsteinium, Mendeleyevium, Fermium,; and some for nuclear research laboratories, such as Berkelium, and Californium.