DALTON'S THOUGHTS ON ATOMIC STRUCTURE

Structure of Atom- The existence of atoms has been proposed since the time of early Indian and Greek philosophers (400 B.C.) who were of the view that atoms are the fundamental building blocks of matter. According to them, the continued subdivisions of matter would ultimately yield atoms which would not be further divisible. The word ‘atom’ has been derived from the Greek word ‘a-tomio’ which means ‘uncut-able’ or ‘non-divisible’. These earlier ideas were mere speculations and there was no way to test them experimentally. These ideas remained dormant for a very long time and were revived again by scientists in the nineteenth century.

The atomic theory of matter was first proposed on a firm scientific basis by John Dalton, a British school teacher in 1808. His theory, called Dalton’s atomic theory, regarded the atom as the ultimate particle of matter (Unit 1). Dalton’s atomic theory was able to explain the law of conservation of mass, law of constant composition and law of multiple proportion very successfully. However, it failed to explain the results of many experiments, for example, it was known that substances like glass or ebonite when rubbed with silk or fur get electrically charged. In this unit we start with the experimental observations made by scientists towards the end of nineteenth and beginning of twentieth century. These established that atoms are made of sub-atomic particles, i.e., electrons, protons and neutrons — a concept very different from that of Dalton.

                                                                      JOHN DALTON

 John Dalton- (born September 5 or 6, 1766, Eaglesfield, Cumberland, England—died July 27, 1844, Manchester), English meteorologist and chemist, a pioneer in the development of modern atomic theory.

Dalton was born into a Quaker family of tradesmen; his grandfather Jonathan Dalton was a shoemaker, and his father, Joseph, was a weaver. Joseph married Deborah Greenup in 1755, herself from a prosperous local Quaker family. Dalton was the youngest of their three ofspring who survived to adulthood.

He attended Johan Flechar's Quaker grammar school in Eaglesfield. When John was only 12 years old, Fletcher turned the school over to John’s older brother, Jonathan, who called upon the younger Dalton to assist him with teaching. Two years later the brothers purchased a school in kendal, where they taught approximately 60 students, some of them boarders.

                                                     MANCHESTER

As a teacher, Dalton drew upon the experiences of two important mentors: Elihu Robinson, a Quaker gentleman of some means and scientific tastes in Eaglesfield, and John Gough, a mathematical and classical scholar in Kendal. From these men John acquired the rudiments of mathematics, greek, and latin. Robinson and Gough were also amateur meteorologists in the Lake District, and from them Dalton gained practical knowledge in the construction and use of meteorologic instruments as well as instruction in keeping daily weather records. Dalton retained an avid interest in meteorologic measurement for the rest of his life.

John Dalton career-In 1793 Dalton moved to Mnchester to teach mathematics at a dissenting academy, the New College. He took with him the proof sheets of his first book, a collection of essays on meteorologic topics based on his own observations together with those of his friends John Gough and Peter Crosthwaite. This work, Meteorological Observations and Essays, was published in 1793. It created little stir at first but contained original ideas that, together with Dalton’s more developed articles, marked the transition of metrology from a topic of general folklore to a serious scientific pursuit.

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.

Postulates of Dalton's Atomic Theory:-

All atoms of a specific element are identical in mass, size, and other properties. However, atoms of different element exhibit different properties and vary in mass and size. Atoms can neither be created nor destroyed. Furthermore, atoms cannot be divided into smaller particles.

According to John Dalton, all matter, whether an element, a compound or a mixture, is composed of tiny particles called atoms.

The following postulates are proposed by John Dalton:

1.All matter is made of very tiny particles called atoms.

2.Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.

3.Atoms of a given element are identical in mass and chemical properties.

• 4.Atoms of different elements have different masses and chemical properties.

• 5.Atoms combine in the ratio of small whole number to form compounds.
• The relative number and kinds of atoms are constant in a given compound.