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Chemistry by committee.

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Atomic weight refers to the averaged mass of the atoms of a chemical element using a scale based on a standard atomic nucleus. Currently the standard is the nucleus of a carbon atom containing six protons and six neutrons (carbon-12). An atomic weight of I corresponds to an average mass equal to one-twelfth the mass of the carbon-12 nucleus. Previous scales were based on hydrogen or oxygen:

1803

John Dalton (top), an English schoolteacher, compiles the first table of atomic weights for various elements, using units where the atomic weight of hydrogen was equal to 1.

1810

Swedish chemist Jons Jacob Berzelius (center) begins work on developing an atomic weight scale based on oxygen equal to 100. But most chemists continue to use the scale based on hydrogen equal to 1.

1860

At a conference held in Karlsruhe, Germany, chemists discuss the need for an improved and consistent atomic weight scale.

1869

Listing the elements in order of atomic weight, Russian chemist Dmitri Mendeleyev (bottom) creates a table in which elements with similar properties fall in the same row (columns in later versions of the table). Mendeleyev's chart becomes known as the periodic table of the elements. A similar table was created at about the same time by the German chemist Lothar Meyer.

1898

A German committee of chemists recommends basing atomic weights on a scale fixing oxygen equal to 16.

1906

The International Committee on Atomic Weights adopts the oxygen equals 16 scale.

1929

Discovery of heavy forms of oxygen (0-17 and 0-18) creates a discrepancy between chemists' atomic weight for oxygen and physicists' atomic mass for the oxygen-16 isotope.

1961

Following approval by the International Union of Pure and Applied Physics and the International Union of Pure and Applied Chemistry, atomic weights are based on the carbon-12 nucleus equals 12 scale.

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Title Annotation:Back Story
Publication:Science News
Article Type:Brief article
Geographic Code:1USA
Date:Jan 29, 2011
Words:309
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