Saturday, September 6, 2014

John Dalton and the Atomic Theory

John Dalton (1766-1844)
On September 6, 1766, English chemist, meteorologist and physicist John Dalton was born. He is best known for his pioneering work in the development of modern atomic theory, and his research into colour blindness. He also recognised that the aurora borealis was an electrical phenomenon.

John Dalton was born into a Quaker family at Eaglesfield, near Cockermouth, Cumberland, England, as son of a handloom weaver. Both he and his brother were born color-blind. After attending a Quaker school in his village in Cumberland, when Dalton was just 12 years old he started teaching there. Around 1790 Dalton seems to have considered taking up law or medicine, but his projects were not met with encouragement from his relatives – Dissenters were barred from attending or teaching at English universities. In 1793, he moved to Manchester and Dalton was appointed teacher of mathematics and natural philosophy at the "New College" in Manchester, a dissenting academy. He remained in that position until 1800, when the college's worsening financial situation led him to resign his post and begin a new career as a private tutor for mathematics and natural philosophy.

For one of his first research projects, Dalton pursued his avid interest in meteorology. He started keeping daily logs of the weather, paying special attention to details such as wind velocity and barometric pressure—a habit Dalton would continue all of his life.[1] He upheld the view, against contemporary opinion, that the atmosphere was a physical mixture of approximately 80 percent nitrogen and 20 percent oxygen rather than being a specific compound of elements. He measured the capacity of the air to absorb water vapor and defined partial pressure in terms of a physical law whereby every constituent in a mixture of gases exerted the same pressure it would have if it had been the only gas present.[2]

Soon after his arrival at Manchester, Dalton was elected a member of the Manchester Literary and Philosophical Society. His first contribution to this society was a description of the defect he had discovered in his own and his brother’s vision. This paper was the first publication on color blindness, which for some time thereafter was known as Daltonism.[2]

In 1803, he calculated atomic weights of chemical elements and assembled them in a table which consisted of six elements namely hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus. He calculated these weights from percentage compositions of compounds using an arbitrary system to determine the probable atomic structure of each compound.[3] John Dalton’s Atomic theory has three principles that remain relatively unchanged. First, Elements are made of the smallest indivisible particles called atoms. Second, all atoms for a particular element are identical. Third, atoms of different elements can be told apart by their atomic weight. Fourth, atoms of different elements can combine in a chemical reaction to form chemical compounds in fixed ratios. Finally, atoms can not be created, destroyed, or divided as they are the smallest particles of matter.[3]

In 1810, Sir Humphry Davy asked Dalton to offer himself as a candidate for the fellowship of the Royal Society, but Dalton declined, possibly for financial reasons. However, in 1822 he was proposed without his knowledge, and on election paid the usual fee. Dalton suffered a minor stroke in 1837, and a second one in 1838 left him with a speech impediment, though he remained able to do experiments. In May 1844 he had yet another stroke; on 26 July he recorded with trembling hand his last meteorological observation. One day later, Dalton fell from his bed and was found lifeless by his attendant.

At yovisto, you can learn more about Dalton's model of the atom in the lecture of MIT Prof. Donald Sadoway on 'Introduction into Solid State Chemistry'.

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