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  • Published on: 7th September 2019
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John Dalton - The Theory of the Atom


John Dalton was born on the 6th of September 1766 into a Quaker family in the settlement of Eaglesfield in the county of Cumberland, England. Early on in life he was educated by his father and then another Quaker who ran a private school. Living in a poor family that couldn't support him, he was forced to get a job at age 10, by 12 he was teaching at a local school and by 14 he was proficient in latin. By the age of 15 Dalton joined his elder brother Jonathan in running a Quaker school 45 miles from their home and around the age of 23 Dalton considered studying law or medicine but his relatives did not encourage it due to religious beliefs and traditions relating to certain universities. When he was 27 he was appointed as the teacher of mathematics and natural philosophy at a struggling college and by 34 he had to resign and started private tutoring in mathematics and natural philosophy. Dalton was a chemist, physicist, and meteorologist. He is best known for his pioneering work in the development of modern atomic theory; and his research into colour blindness, sometimes referred to as Daltonism, in his honour. He was never married and only had a few close friends. All in all he lived a modest and unassuming personal life. In the 7 years before his death he suffered from 3 strokes and on the 27th of July 1844, in Manchester, Dalton fell from his bed and died.

Dalton’s Atomic Theory:

Experiments with gases first became possible in the 19th century and led John Dalton to propose a modern theory of the atom based on the following:

1: Matter is made up of atoms that are indivisible and indestructible.

2: All atoms of an element are identical.

3: Atoms of different elements have different weights and different chemical properties.

4: Atoms of different elements combine in simple whole numbers to form compounds.

5: Atoms can’t be created or destroyed. When a compound decomposes, the atoms are recovered and unchanged.

He performed a series of experiments on mixtures of gases to determine what effect properties of the individual gases had on the properties of the mixture as a whole. While trying to explain the results of those experiments, Dalton developed the hypothesis that the sizes of the particles making up different gases must be different. Dalton’s law of partial pressures states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the component gases:

 Dalton measured the amount of water vapour that air could absorb at different temperatures.

Then derived the formula: PT = P1 + P2 + P3 + … (known as Dalton’s law of Partial Pressures.

J. J Thompson -


Joseph John Thomson, who was always called J.J., was born in Cheetham Hill, England, near Manchester, in 1856. His father was a bookseller who planned for Thomson to be an engineer. When an apprenticeship at an engineering firm couldn't be found, Thomson was sent to bide his time at Owens College at the age of 14. In 1876, he received a small scholarship to attend Trinity College at Cambridge to study mathematics.

Thomson worked in the Cavendish Laboratory after graduation, under the tutelage of Lord Rayleigh. He quickly earned a membership in the prestigious Royal Society and was appointed Rayleigh’s successor as the Cavendish Professor of Physics at the age of 28. He was both respected and well-liked, and students came from around the world to study with him.

Thomson determined that all matter is made up of tiny particles that are much smaller than atoms. He originally called these particles 'corpuscles,' although they are now called electrons. This discovery upended the prevailing theory that the atom was the smallest fundamental unit.

In 1906, Thomson began studying positively charged ions, or positive rays. This led to one of his other famous discoveries in 1912, when he channeled a stream of ionized neon through a magnetic and an electric field and used deflection techniques to measure the charge to mass ratio. In doing so, he discovered that neon was composed of two different kinds of atoms, and proved the existence of isotopes in a stable element. This was the first use of mass spectrometry.

Thomson married Rose Paget, one of his students, in 1892. They had one daughter, Joan, and one son, George Paget Thomson, who went on to become a physicist and win a Nobel Prize of his own. J.J. Thomson published 13 books and more than 200 papers in his lifetime. In addition to being awarded the Nobel Prize in 1906, he was knighted in 1908 by King Edward VII. He left research in 1918 to become Master of Trinity College. He died in Cambridge on August 30, 1940, and is buried in Westminster Abbey near two other influential scientists: Isaac Newton and Charles Darwin.

In 1894, Thomson began studying cathode rays, which are glowing beams of light that follow an electrical discharge in a high-vacuum tube. It was a popular research topic among physicists at the time because the nature of cathode rays was unclear.

Thomson devised better equipment and methods than had been used before. When he passed the rays through the vacuum, he was able to measure the angle at which they were deflected and calculate the ratio of the electrical charge to the mass of the particles. He discovered that the ratio was the same regardless of what type of gas was used, which led him to conclude that the particles that made up the gases were universal.

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