Posted on 10/24/2003 8:58:34 AM PDT by balrog666
Bicentenary of atomic weights John Dalton presented chemistry milestone 200 years ago today. 21 October 2003
PHILIP BALL
Dalton was the first to draw chemical elements. © SPL
Two hundred years ago today, British chemist John Dalton helped turn chemistry into an exact science. He presented his Table of Atomic Weights at the Manchester Literary and Philosophical Society.
Dalton (1766-1844) is widely regarded as the founder of the idea that all matter is made of tiny, indivisible particles called atoms. Although atoms were proposed 2500 years ago in ancient Greece, Dalton's work made them an indispensable part of chemical theory.
Today's chemists can see and manipulate individual atoms. For Dalton's contemporaries, such notions would have been absurd. Indeed, because atoms were not directly observable, some scientists refused to believe in them for more than 100 years after Dalton's table appeared. It wasn't until 1908 that French physicist Jean Perrin produced experimental evidence of their existence.
Until then, Dalton's atoms were regarded largely as a convenient explanatory tool for chemists. For example, one of the pupils at a college close to where the Mancunian Quaker taught concluded: "atoms are square blocks of wood invented by Mr Dalton."
Dalton was not in the mainstream of nineteenth-century chemical science. He was the archetypal scientific dabbler, with interests ranging from meteorology to colour vision - he was colour-blind, a condition that became known as daltonism. He taught chemistry but had no experience of chemical research.
Perhaps this gave him the confidence to take a step at once bold and simple: he drew atoms. He proposed that chemical elements could be depicted as circular symbols denoting their constituent atoms, so that compounds could be illustrated as little clusters of these symbols.
This was a convenient shorthand for chemists and an explanation for the way that elements generally seem to combine in fixed proportions. For example, hydrogen and oxygen gases combine to make water in a 2:1 ratio. This makes sense, Dalton argued, if molecules, or 'compound particles' as he called them, consist of fixed quotients of atoms.
Chemists at that time could measure the weight ratios of different elements in a compound. To convert such measurements to ratios of atoms, they needed to know the weight of each kind of atom. Hydrogen has the lightest atoms, so Dalton used fellow chemists' measurements to deduce the weights of other atoms relative to hydrogen.
This he unveiled in his Table of Atomic Weights. Many of his numbers were wrong: for example, he thought that oxygen atoms were eight times heavier than hydrogen; they are, in fact, 16 times as heavy. Later, more careful chemists - in particular, the Swede Jöns Jacob Berzelius - corrected these mistakes.
Dalton published his full atomic theory in a book called A New System of Chemical Philosophy in 1808. As much an educator as a scientist, he was a co-founder of the British Association for the Advancement of Science.
He was also president of the Manchester Literary and Philosophical Society, to which he first presented his table on 21 October 1803. The society is currently celebrating the bicentennial with two weeks of events.
© Nature News Service / Macmillan Magazines Ltd 2003
Speculative Natural History, not real science.
LOL, totally false!
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