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Caesium is an alkali metal found in group 1, period 6 of the periodic table. General properties: Symbol: Cs Number: 55 Atomic Weight: 132.9054519 Electrons per shell: 2, 8, 18, 18, 8, 1 Physical Properties: Melting point: 28.44 ° [|C] Boiling point: 671 ° [|C] Density near r.t: 1.93 [|__g/cm__][|__−3__] ** Brief Description:  ** Caesium is a silvery gold alkali metal that is soft and ductile, characterised by a spectrum containing two bright lines in the blue. Along with bromine, mercury, gallium, francium, rubidium, caesium is one of the six metals that are liquid at (or near) room temperature. (Note that francium and rubidium are of the same group as caesium, and thus naturally share some properties) Being of group 1, Caesium shares many similar physical and chemical properties similar to those of rubidium and potassium (also from group 1). As such, it is also extremely reactive and pyrophoric, reacting explosively with cold water (as seen from the videos), even with ice at temperatures of -116°C and above. It is the least electronegative element that has stable isotopes (only caesium-133). (Note: Electronegativity is a chemical property that describes the tendency of an atom to attract electrons towards itself to form negative ions. The opposite of electronegativity is electropositivity, a measure of an element’s ability to lose electrons. As such, caesium is also the most electropositive element.)It is mined mostly from pollucite, while radioisotopes (atoms with an unstable nucleus – with excess energy) are extracted from waste produced by nuclear reactors. ** Discovery and uses:  ** Caesium was discovered in 1860 by two German chemists, Robert Bunsen and Gustav Kirchhoff by flame spectroscopy. Flame emission spectroscopy is a process whereby a sample of a material is brought into the flame as a gas or sprayed solution. The heat from the flame evaporates the solvent and breaks chemical bonds to create free atoms. The thermal energy also excites the atoms into excited electronic states that subsequently emit light when they return to the ground electronic state. Each element emits light at a characteristic wavelength, which is dispersed by a grating or prism and detected in the spectrometer. As such, caesium was discovered and is characterized by it’s spectrum containing two bright lines of blue. From then on, caesium has been used as a “getter” (a deposit of reactive material that is deliberately placed inside a vacuum system, for the purpose of completing and maintaining the vacuum) in vacuum tubes and in photoelectric cells. However, in 1967, a specific frequency from the emission spectrum of caesium-133 was chosen to be used in the definition of a second, and caesium has been widely used in atomic clocks since then. (An atomic clock is a clock that uses an elctronic transition frequency in the microwave region of the electromagnetic spectrum of atoms as a frequency standard for it’s timekeeping element. As such, atomic clocks are among the most accurate time and frequency standards known.) Since the 1990s, the largest application of the element has been as caesium formate for drilling fluids (a fluid used to frill boreholes into the earth). It has a range of applications in the production of electricity, in electronics, and in chemistry. ** References:  ** [] [] [] [] [] [] [] [] []