Cesium Facts: Atomic Number 55 or Cs

Cesium or caesium is a metal with the element symbol Cs and atomic number 55. This chemical element is distinctive for several reasons. Here is a collection of cesium element facts and atomic data:

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Cerium Facts - Ce or Atomic Number 58

By Anne Marie Helmenstine, Ph.D.

Cesium Element Facts

• Gold is often listed as the only yellow-colored element. This is not exactly true. Cesium metal is silvery-gold. It is not as yellow as high-karat gold but has a warm color
• Although not a liquid at room temperature, if you hold a vial containing cesium in your hand, your body heat will melt the element into its liquid form, which resembles pale liquid gold.
• German chemists Robert Bunsen and Gustav Kirchhoff discovered cesium in 1860 when analyzing the spectrum of mineral water. The name for the element comes from the Latin word "caesius", which means "sky blue". This refers to the color of the line in the spectrum the chemists saw that tipped them off about the new element.
• Although the official IUPAC name for the element is cesium, several countries, including England, retain the original Latin spelling of the element: caesium. Either spelling is correct.
• Samples of cesium are kept in sealed containers, under an inert liquid or gas or in a vacuum. Otherwise, the element would react with air or water. The reaction with water is much more violent and energetic than the reaction between water and other alkali metals (e.g., sodium or lithium). Cesium is the most alkaline of the elements and reacts explosively with water to produce cesium hydroxide (CsOH), a strong base that can eat through glass. Cesium spontaneously ignites in air.
• Although francium is predicted to be more reactive than cesium, based on its location on the periodic table, so little of the element has been produced no one knows for sure. For all practical purposes, cesium is the most reactive metal known to man. According to the Allen scale of electronegativity, cesium is the most electronegative element. Francium is the most electronegative element according to the Pauling scale.
• Cesium is a soft, ductile metal. It is readily drawn into fine wires.
• Only one stable isotope of cesium occurs naturally — cesium-133. Numerous artificial radioactive isotopes have been produced. Some radioisotopes are produced in nature by slow neutron capture within old stars or by the R-process in supernovae.
• Non-radioactive cesium is not a nutritional requirement for plants or animals, but it's not particularly toxic, either. Radioactive cesium presents a health hazard because of the radioactivity, not chemistry.
• Cesium is used in atomic clocks, photoelectric cells, as a catalyst to hydrogenate organic compounds, and as a 'getter' in vacuum tubes. The isotope Cs-137 is used in cancer treatments, to irradiate foods, and as a tracer for drilling fluids in the petroleum industry. Nonradioactive cesium and its compounds are used for infrared flares, to make specialty glasses, and in beer brewing.
• There are two methods used to prepare pure cesium. First, the ore is sorted by hand. Calcium metal may be combined with fused cesium chloride or electric current may be passed through a molten cesium compound.
• Cesium is estimated to be present at an abundance of 1 to 3 parts per million in the Earth's crust, which is a fairly average abundance for a chemical element. One of the richest sources of pollucite, an ore that contains cesium, is the Tanco Mine at Bernic Lake in Manitoba, Canada. Another rich source of pollucite is the Karibib Desert in Namibia.
• As of 2009, the price of 99.8% pure cesium metal was around $10 per gram or $280 per ounce. The price of cesium compounds is much lower.

Cesium Atomic Data

• Element Name: Cesium
• Atomic Number: 55
• Symbol: Cs
• Atomic Weight: 132.90543
• Element Classification: Alkali Metal
• Discoverer: Gustov Kirchoff, Robert Bunsen
• Discovery Date: 1860 (Germany)
• Name Origin: Latin: coesius (sky blue); named for the blue lines of its spectrum
• Density (g/cc): 1.873
• Melting Point (K): 301.6
• Boiling Point (K): 951.6
• Appearance: extremely soft, ductile, light gray metal
• Atomic Radius (pm): 267
• Atomic Volume (cc/mol): 70.0
• Covalent Radius (pm): 235
• Ionic Radius: 167 (+1e)
• Specific Heat (@20°C J/g mol): 0.241
• Fusion Heat (kJ/mol): 2.09
• Evaporation Heat (kJ/mol): 68.3
• Pauling Negativity Number: 0.79
• First Ionizing Energy (kJ/mol): 375.5
• Oxidation States: 1
• Electronic Configuration: [Xe] 6s1
• Lattice Structure: Body-Centered Cubic
• Lattice Constant (Å): 6.050