Found in the first column of the periodic table of elements, the alkali metals react readily with many substances in the environment, even with relatively inert substances, such as water. Because of their extreme reactivity, alkali metals rely on special containment to prevent inadvertent reactions that tarnish the purity of the sample. Alkali metals share certain identifying characteristics: extreme reactivity, pliability, silvery colour and low density.
Caesium, chemical symbol Cs, identifies with atomic number 55. Because of its low melting point, caesium, an extremely soft and waxlike substance, is one of three metals that exist as a liquid at room temperature. It reacts violently with cold water to form caesium hydroxide, an alkali or base, and hydrogen gas. First isolated in 1860, caesium is expensive and difficult to produce because of its high reactivity to everyday substances, such as air and water. One radioactive form of caesium, caesium-137, is an important ingredient in cancer treatment. One special function of caesium occurs in the highly accurate atomic clocks in which oscillations of electrons -- negatively charged particles -- in the outermost shell of the caesium atom dictate time.
Francium, chemical symbol Fr and atomic number 87, is the most highly reactive metal on the periodic table. Much of what scientists know about francium stems from educated guesses rather than based on actual observations or experiments conducted. Francium should appear similar to other alkali metals, namely, silvery-white and soft, maybe even existing as a liquid at room temperature. Hypothetically, francium would react explosively with water in a comparable way to caesium but much more quickly. In reality, scientists could not perform this type of experiment because francium is rare, expensive and dangerously radioactive.
Lithium, or Li, has an atomic number of 3, making it the lightest of all alkali metals. Although the least reactive in the family, lithium still reacts vigorously in the presence of water. Natural sources of this whitish-silver metal include petalite and spodumene, both minerals. Brine, or water that contains a high percentage of lithium carbonate, supplies the main source of lithium in the world. The U.S. produces only 760,000 tons of lithium annually, but it consumes most of the lithium produced in the world, according to the Mineral Information Institute. Lithium batteries may replace traditional batteries in certain applications. Medication based on lithium can treat mental illness. Manufacturers use the metal in the production of glass and rubber.
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Potassium, chemical symbol K with an atomic number of 9, readily reacts with air, causing its surfaces to take on a tarnished appearance. When potassium meets water, a purple-tinted, violent explosion occurs, giving off heat. Hydrogen gas and a base, potassium hydroxide, form. In comparison, potassium reacts slower than rubidium but faster than sodium. Similar to other members of the alkali metals, potassium is soft, readily cut by a knife. Potassium resides at the interior of meteorites, within the Earth's crust and inside the human body.
A silvery-white metal, rubidium, or Rb, holds place number 37 on the periodic table. As with other alkali metals, rubidium cannot exist alone in nature and explosively reacts with both air and water. Discovered in 1861, rubidium readily forms compounds with many other elements, such as oxygen and nitrogen. Exceptional storage conditions and specially designed containers prevent or limit rubidium's activity with substances in its environment. Because of its similarities to caesium, rubidium may replace caesium for certain industrial uses. Rubidium, in its radioactive form, has a significant role in medicine to trace blood flow in suspected cases of blockage and to treat seizures and goitre, an abnormal condition of the thyroid gland located in the neck region.
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Sodium, chemical symbol Na with an atomic number of 11, is a silver-hued metal you can slice with a knife. Pure sodium does not exist by itself in nature because it combines readily with a variety of substances, including chlorine to form table salt. Sodium reacts vigorously with water to form the base sodium hydroxide and hydrogen gas. To keep pure sodium from reacting with oxygen and other substances in the air, you must store it under liquid paraffin. Sodium, first discovered by Sir Humphrey Davy in 1807, is vital in the manufacture of sodium cyanide and sodium peroxide and can act as a coolant in certain nuclear reactors.