Metal

Most reactive: Caesium
Caesium (Cs), chemical element of Group I of the periodic table, the alkali metal group, first element to be discovered spectroscopically (1860), by Robert Bunsen and Gustav Kirchhoff, who named it for the unique blue lines of its spectrum (Latin caesius, “sky-blue”). This silvery white metal is the most reactive and one of the softest of all metals and is liquid at room temperature. It is about half as abundant as lead and 70 times as abundant as silver. Cesium occurs in minute quantities in the Earth's crust, as in the minerals pollucite and lepidolite. Cesium can be isolated by electrolysis of a molten cesium-barium-cyanide mixture and by other methods. Cesium reacts explosively with cold water; it readily combines with oxygen, so that it is used in electron tubes as a “getter” to clear out the traces of oxygen and other gases trapped in the tube when sealed.

http://www.britannica.com/eb/article-9022171/cesium

More information:
http://en.wikipedia.org/wiki/Caesium


Least reactive: Gold

Gold is a chemical element with the symbol Au (from the Latin aurum, meaning shining dawn) and atomic number 79.

Chemically, gold is a trivalent and univalent transition metal. Gold does not react with most chemicals, but is attacked by chlorine, fluorine, aqua regia and cyanide. Gold dissolves in mercury, forming amalgam alloys, but does not react with it. Gold is insoluble in nitric acid, which will dissolve silver and base metals, and this is the basis of the gold refining technique known as "inquartation and parting".

http://en.wikipedia.org/wiki/Gold

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