David Vollan, age 12, of Midway City, California, for his question:

What exactly is barium?

Barium is one of the earth's 92 chemical elements and the best way to study the nature of an element is to begin with its basic atom. Then we locate and consider its related family of elements on the Periodic Table. We now have the key information that makes the behavior of barium and the compounds it forms seem logical.

The element barium has the atomic number 56 because its basic atom is the only atom with 56 protons in its nucleus. These positive particles are balanced with a normal quota of negative electrons that orbit around the nucleus in five complete shells plus a partial outer shell of two electrons. The atomic weight of our element is 137.34. On the Periodic Table of chemical elements, barium is charted in a family of grey metals called the "alkaline earths." This odd family name arose because all the metallic brethren were first isolated from earthy minerals.

Beryllium, magnesium and strontium were isolated in the 1700s. Barium and calcium were found in 1808 and the family, of six was completed when radium was discovered in 1898. Their atom sizes range from little berillium, atomic number 4, to radium, atomic number 88. But each of the six basic atoms in the group has a closely knit structure with an incomplete outer shell of two electrons. This is the feature that unites the six alkaline earths in a related family of chemical elements, even though their differences appear to outweigh their similarities.

Calcium, for example, is the fifth most plentiful element in the earth's crust and radium is the sixth rarest. And calcium is 186 times more plentiful than its brother barium. The name "barium" means "heavy" and its density is more than twice that of its lightweight brother, magnesium. Five of the brothers are fairly stable, but radium, the rare one, is highly radioactive, and the isotope strontium 90 is a dangerous item of radioactive fallout. It penetrates and replaces the calcium of the bones and fatally damages the vital marrow of the skeleton.

This basic background material, you might say, is the meat course of our barium study. The dessert is its practical usefulness to us. A portion of the dark grey element is used in alloys to add hardness and toughness. Alloys of nickel and barium are used in vacuum tubes and spark plugs. But most of barium's usefulness is in the  assorted compounds that its outer electron shell is willing to form with other atoms. A trace of barium nitrate adds vivid green flashes to fireworks. Barium sulphide adds a glow to luminous paints and barium chromate is used on matches. A watery drink of barium sulphate is given to patients needing internal investigation. The intestines do not absorb the milky fluid and it blocks X rays. It helps to give the doctor a clear picture of what goes on inside.

Barium dioxide is used to make the germicide bleach, hydrogen peroxide. And barium monoxide is used in sugar refining. The most common barium ore is barite, a natural compound of barium, sulphur and oxygen. Barite is combined with zinc to make quantities of lithopone. And lithopone is a bland filler used in rubber and linoleum and also added to give a glossy gleam to paints, glass and polished paper.

Barium alone never occurs in nature and we can extract it only from a few min¬eral ores. The United States mines more than 100,000 tons of barite ore every year. Some of it is used to yield barium metal. Most of it is merely refined for use as multi purpose barium compounds. Mines are scattered through the eastern and a few central states and a few are in the western prairies. In the Dakotas and Oklahoma, rose shaped, rose tinted barite stones may be found on sandy soils. In Missouri we may find glassy barite crystals of pastel pinks, yellows and greens in flat or bar shapes or in tight little fists of jagged, icy looking splinters.