NCERT Chemistry Notes Class 11 Discovery Of Electrons And Thomson’s Model Of Atom
CBSE Class 11 – Chemistry Notes – Unit 2 – Structure of Atom – DISCOVERY OF ELECTRONS AND THOMSON’S MODEL OF ATOM
Before discussing the topic, we will understand in brief about the origin of the word “atoms” and Dalton’s Atomic Theory. The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos” means indivisible).
He believed that atoms were indivisible and indestructible.
His ideas did agree with the later scientific theory, but could not explain the chemical behavior of atoms.
Later, John Dalton (England 1766-1844) who was a school teacher, studied the ratios in which elements combine in chemical reactions. He formulated first Modern Atomic Theory. John Dalton postulated what was known about chemical reactions at that time and proposed the first atomic model. He explained;
– Conservation of Mass
– Law of Multiple Proportions
– Law of Definite Composition
Dalton combined the observations into one theory which stated that all matter was composed of small indivisible particles that he called atoms.
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Dalton’s Atomic Theory
1) All elements are composed of tiny indivisible particles called atoms.
2) Atoms of the same element are identical. Atoms of any one element are different from those of any other element.
3) Atoms of different elements combine in simple whole -number ratios to form chemical compounds. E.g. CO2
4) In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.
5) Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties of that element.
Dalton atomic theory suggested that atoms are indivisible (could not be broken into smaller particles. But the discovery of subatomic particles inside an atom disproved this postulate of Dalton atomic theory. After the discovery of these subatomic particles; it became necessary to find out how these particles are arranged inside the atoms.
Sir Joseph John Thomson (Sir J.J Thomson) was an English physicist, who helped revolutionize the knowledge of atomic structure by his discovery of the electron (1897). He received the Nobel Prize in Physics in 1906 and was knighted in 1908.
Several scientists, such as William Prout and Norman Lockyer, had suggested that atoms were built up from a more fundamental unit, but they envisioned this unit to be the size of the smallest atom, hydrogen. Thomson in 1897 was the first to suggest that one of the fundamental units was more than 1,000 times smaller than an atom, suggesting some subatomic particle. Thomson discovered this through his explorations on the properties of cathode rays that led to the discovery of negatively charged particles, now known as electrons.
The CRT(Cathode Ray Tube) consists of several elements, starting with a tube that’s vacuum sealed to keep air out of it. On one side of the inside of the tube, there’s a cathode and an anode. The cathode is a negatively-charged conductor, and the anode is a positively-charged conductor. The rays coming from the cathode are attracted towards the anode. A small hole in the anode allows the beam to pass through it; on the opposite side of the tube is a coating that glows when the beam falls on it. This allowed J. J. Thomson to see where the electron beam was hitting. Of course, before his experiment, we didn’t know electrons existed. So, no one was calling it an electron beam. Instead, what flowed off the cathode toward the anode was called ‘cathode rays.’ Hence the name cathode ray tube.
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Thomson made his suggestion on 30 April 1897 following his discovery that cathode rays (at the time known as Lenard rays) could travel much further through the air than expected for an atom-sized particle. He estimated the mass of cathode rays by measuring the heat generated when the rays hit a thermal junction and comparing this with the magnetic deflection of the rays. His experiments suggested not only that cathode rays were over 1,000 times lighter than the hydrogen atom, but also that their mass was the same in whichever type of atom they came from. He concluded that the rays were composed of very light, negatively charged particles which were a universal building block of atoms. He called the particles “corpuscles”, but later scientists preferred the name electron which had been suggested by George Johnstone Stoney in 1891, prior to Thomson’s actual discovery.
In April 1897, Thomson had only early indications that the cathode rays could be deflected electrically. A month after Thomson’s announcement of the corpuscle, he found that he could reliably deflect the rays by an electric field if he evacuated the discharge tube to a very low pressure. By comparing the deflection of a beam of cathode rays by electric and magnetic fields he obtained more robust measurements of the mass-to-charge ratio that confirmed his previous estimates. This became the classic means of measuring the charge and mass of the electron. By carefully measuring how the cathode rays were deflected by electric and magnetic fields, Thomson was able to determine the ratio between the electric charge (e) and the mass (m) of the rays. Thomson’s result was
e/m = 1.8 10-11 coulombs/kg
Thomson believed that the corpuscles emerged from the atoms of the trace gas inside his cathode ray tubes. He thus concluded that atoms were divisible and that the corpuscles were their building blocks.Thus, in 1904 Thomson suggested a model of the atom.
According to Thomson “plum pudding” model of an atom:
1) An atom consists of a sphere of positive charge with negatively charged electrons embedded in it
2) The positive and negative charges in an atom are equal in magnitude, due to which an atom is electrically neutral. It has no overall positive and negative charge.
Limitations of Thomson’s atomic model
Thomson’s model of atom failed to explain how a positive charge holds the negatively charged electrons in an atom. Hence it could not explain the stability of an atom. This theory also failed to account for the position of the nucleus in an atom. Although Thomson’s model was able to explain the overall neutrality of the atom but was not consistent with the results of later experiments.
Thomson was awarded Nobel Prize for physics in 1906, for his theoretical and experimental investigations on the conduction of electricity by gases.
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