Today (September 6, 1766) is the
birthday of John Dalton, who first published a list of modern atomic theories
and atomic weights.
John Dalton was born on September
6, 1766, in Eaglesfield, near Cockermouth, Cumberland, England, to a Quaker
family. His father was a weaver. He received his early education from his
father, Quaker John Fletcher, who ran a private school in the nearby village of
Partsha Hall. Dalton's family was too bad to support him for a long. And from the
age of ten, he began to earn his living in the service of the wealthy local
Quaker Elihu Robinson. In 1800, at the age of 34, he became secretary of the
Manchester Literary Philosophical Society. The following year, he
published his major research papers on the components of gases, such as vacuum
and atmosphere, the pressure of steam and other gases at different
temperatures, and so on.
Dalton studied the pressure of
vapor at several points in the range of 0–100 (C (32 - 212 F) and proposed the
theory that the vapor pressure of all liquids would be equal at equal
temperature changes, observing the vapor pressure of many different liquids.
Foremost among Dalton's discoveries was the atomic theory in chemistry. The
doctrine, however, is deeply ingrained in his name, but the connection is not
fully understood. The main features of Dalton's atomic theory are: All elements
are made up of tiny particles called atoms. The atoms of a particular element
all have the axis's exact size, mass, and properties. Atoms of different
elements may be different from these.
The atom cannot be created,
destroyed, or perforated. Atoms of different elements mix in whole proportions
to form a chemical compound. In chemical reactions, atoms combine, separate, or
modify. Dalton published the first list of atomic weights. It contained six
elements: hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus. It was
constructed from the assumption of weight 1 for a hydrogen atom. Not much is
known about how he first discovered this. But in his laboratory notebook, on
September 6, 1803, there is a list of several atomic weights from a study of
water, ammonia, and carbon dioxide.
In his first extended discussion
of atomic theory (1808), Dalton proposed an additional (controversial) rule of
"great simplicity". This rule could not be independently confirmed.
But some such assumptions were necessary to propose formulas for some simple
molecules. Based on the calculation of the atomic weights on which this rule is
based, the atoms of two different elements form only one mixture, such as
water, which produces hydrogen and oxygen, or ammonia, which has hydrogen
and nitrogen. The molecules of that compound are considered to contain one atom
of each element. For components known at the time, such as two oxides of carbon
or three oxides of nitrogen, their combinations were considered as simple
as possible. For example, if two such combinations are known, one must contain
an atom of each element, and the other must contain one atom of one element and two
atoms of the other.
This is an assumption derived
from a belief in the simplicity of nature. Scientists have no evidence to
determine how many atoms of each element combine to form molecules. But this or
some other rule is absolutely necessary for any initial theory. This is because
one needed a molecular formula to calculate atomic weights. He considered
Dalton's "greatest simplicity rule", the procedure for water OH
and ammonia NH, utterly different from our modern understanding (H2O, NH3).
On the other hand, his rule of simplicity led him to propose modern formulas
suitable for two carbon oxides (CO and CO2). Despite the uncertainty at the
center of Dalton's nuclear theory, the principles of the theory survived.
Shortly after arriving in
Manchester in 1794, Dalton was elected a member of the "Lit &
Phil" Literary and Philosophical Society of Manchester. A few weeks later, he wrote his first article, "Unusual Facts About Color Vision," where
he noted a lack of color perception due to the discoloration of the liquid
medium of eye vision. Since he and his brother were color blind, he realized
that this condition must be hereditary. John Dalton, best known for advancing
modern nuclear theory, the study of color blindness, and gases and
liquids, passed away on July 27, 1844, in Manchester, Lancashire, England, at 77.
Source By: Wikipedia
Information: Ramesh, Assistant
Professor of Physics, Nehru Memorial College, Puthanampatti, Trichy.
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