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The s-block of the periodic table of elements consists of the first two groups: the alkali metals and alkaline earth metals, plus hydrogen and helium.

These elements are distinguished by the property that in the atomic ground state, the highest-energy electron is in an s-orbital. Except in hydrogen and helium, these electrons are very easily lost to form positive ions. The helium configuration is chemically exceedingly stable and thus helium has no known stable compounds; thus it is generally grouped with the noble gases.

The other elements of the s-block are all extremely powerful reducing agents, so much so that they never occur naturally in the free state. The metallic forms of these elements can only be extracted by electrolysis of a molten salt, since water is much more easily reduced to hydrogen than the ions of these metals. Sir Humphry Davy, in 1807 and 1808, was the first to isolate all of these metals except lithium, beryllium, rubidium and caesium. Beryllium was isolated independently by F. Wooler and A.A. Bussy in 1828, while lithium was isolated by Robert Bunsen in 1854, who isolated rubidium nine years later after having observed it and caesium spectroscopically. Caesium was not isolated until 1881 when Carl Setterberg electrolysed the molten cyanide.

The s-block metals vary from extremely soft (all the alkali metals) to quite hard (beryllium). With the exception of beryllium and magnesium, the metals are too reactive for any structural use except as very minor components (<2%) of alloys with lead. Beryllium and magnesium, though very expensive, are valuable for uses that require strength and lightness. They are extremely valuable as reducing agents to extract titanium, zirconium, thorium and tantalum from their ores, and have other uses as reducing agents in organic chemistry.

All the s-block metals are dangerous fire hazards which require special extinguishants to extinguish, except for beryllium and magnesium, storage must be under either argon or an inert liquid hydrocarbon. They react vigorously with water to liberate hydrogen, except for magnesium, which reacts slowly, and beryllium, which reacts only when amalgamated with mercury to destroy the oxide film. Lithium has similar properties to magnesium due to the diagonal relationship with magnesium in the periodic table.

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