Barium

Barium is a soft, silvery alkaline earth metal in group 2, below strontium and above radium. Its configuration, [Xe] 6s2, gives two easily lost outer electrons, so barium forms Ba^2+ ions in almost all its compounds. Going down group 2 the outer electrons are farther from the nucleus and easier to remove, which makes barium more reactive than calcium or strontium: it tarnishes quickly in air, reacts vigorously with water, and is stored under oil. The name reflects the density of its minerals, especially baryte, barium sulfate, which is unusually heavy for a pale, non-metallic-looking stone.

Barium's compounds range from harmless to deadly depending on solubility. Barium sulfate is so insoluble that it passes through the gut untouched, and because barium's 56 electrons absorb X-rays strongly, a 'barium meal' outlines the oesophagus, stomach and intestines on X-ray images. Soluble barium salts, by contrast, are poisonous because Ba^2+ blocks potassium channels in muscle and nerve cells. Most baryte is ground up and added to drilling mud to weigh it down. Barium compounds also colour fireworks green, form permanent ferrite magnets, and appear in the high-temperature superconductor YBCO.

Discovery

In the early 1600s, Vincenzo Casciarolo, a shoemaker and alchemist in Bologna, found that heated pebbles of baryte glowed in the dark after exposure to light; these became famous as 'Bologna stones'. In 1772 Carl Wilhelm Scheele recognised a new earth, baryta (barium oxide), in baryte. Humphry Davy isolated barium metal in 1808 by electrolysis of molten barium hydroxide, using the new electric battery. The name comes from the Greek barys, meaning 'heavy'.