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Discovery of electron-experimental setup, observation, conclusion, solved examples and FAQ’s

Did you know that metals conduct electricity? 

Back in the early 1800s, this peculiar behaviour puzzled scientists from all over the world. According to Dalton, all matter was made up of tiny indivisible atoms and this theory dominated science for almost the next 100 years. The electrical properties in the matter were a subject of deep speculation and interest. The work of those people investigating these properties led to the culmination of J.J. Thomson in the discovery of electrons. His experiment on cathode ray discharge tubes (1897) confirmed the existence of particles smaller than atoms. 

J.J Thomson was continuously working on the conduction of electricity in gas. Before the discovery of electrons, researchers used to think molecules are neutral and once broken, they form ions which are responsible for conduction but experimental data of cathode rays forced Thomson to think to explore more and ultimately figured out that an atom is not a fundamental particle. 

J.J Thomson (in 1906) won the Nobel prize for his long remarkable work on the conduction of electricity in gasses. He revolutionized the knowledge of atomic structure by his discovery of the electron & stands out as the starting point for the research into the structure of the atom that has dominated 20th-century science. 


Table of contents

What is an electron?

Electrons are fundamental stable subatomic particles. It has a negative charge equal to imageThe mass of an electron is imagetimes the mass of a proton (mass of a proton is image

Experimental setup used for discovery of electron


Cathode ray discharge tube: It is a cylindrical hard glass tube that is fitted with two metallic electrodes (Anode and Cathode) connected to a battery. The gas taken in the discharge tube was subjected to a very low pressure (~0.0001 atm) maintained by a vacuum pump and high voltage (~10,000 volts) to generate the cathode rays.

Cathode ray: A cathode ray is a stream of electrons observed in a discharge tube. Cathode rays are so named because they are emitted by the negative electrode (cathode) in a discharge tube. 

Observations

  • Cathode rays move from cathode to anode.
  • Cathode rays travel in a straight line in the absence of electric and magnetic fields.
  • Cathode rays are observed with the help of fluorescent screens.
  • Cathode rays get deflected when placed in an electric field and magnetic field. The direction of deflection shows that they are negatively charged.



  • Cathode rays rotate a light paddle wheel placed in its path.

  • Cathode rays cast the shadow of an object on the ZnS screen placed in its path.

Conclusion 

The above observations led to the conclusion that:

  • Cathode rays consist of negatively charged particles. 
  • These charged particles were identified as corpuscles, later renamed electrons. 
  • Cathode rays have identical properties regardless of the elements used to produce them.
  • Electrons are fundamental particles. Electrons are in fact the basic constituent of all the atoms.
  • Thomson disproved Dalton’s atomic theory because according to Dalton’s atomic theory atoms were the smallest unit of matter and they were indivisible. So, the presence of electrons fails this statement of Dalton’s atomic theory.

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Practise Problems:

Q1. It was concluded from the cathode ray discharge tube experiment that:

(A) mass of an electron is fractional.
(B) matter contains electrons.
(C) matter contains the nucleus.
(D) None of the above

Answer: (B)

Solution: Cathode rays consist of negatively charged particles. These charged particles were identified as corpuscles, later renamed electrons and electrons are the basic constituents of all the atoms.

Q2. Which of the following statements is true for cathode rays? 

(A) It is not deflected by a magnetic field.
(B) It does not travel in a straight line.
(C) It is not deflected by an electric field.
(D) It consists of only negative particles.

Answer: (D)

Solution: In the presence of electric and magnetic fields, the direction of deflection shows that a cathode ray is a stream of negatively charged particles called electrons.

Q3. Electrons are _____ charged. 

(A) positively 
(B) negatively
(C) None of these
(D) Yet to be determined

Answer: (B)

Solution: charge of electron = image

Q4. Cathode rays travel in_______from cathode to anode, in absence of electric or magnetic field. 

(A) parabolic path
(B) circular path
(C) straight line
(D) all of these

Answer: (C)

Solution: Cathode rays travel in a straight line from cathode to anode, in absence of electric or magnetic field.

Frequently Asked Questions-FAQ

Question 1. Was Thomson the first person who performed a discharge tube experiment?
Answer: No, many people performed this experiment and observed the same rays, but they were not able to conclude the property of the observed rays, few famous names were Perrin and Hertz. Hertz also applied an electric field to the discharge tube experiment but due to some experimental error, he was not able to observe any deflection. Some researchers say Hertz's experimental failure gives an idea to Thomson. Thomson introduced some quantity of air between the electrodes in the discharge tube and performed this experiment. 

Question 2. Why is the low pressure of gas required in the cathode ray discharge tube experiment?
Answer: At low pressure, the amount of gas within the discharge tube is less thereby electrons of the cathode ray will experience a very less number of collisions with the ionized gas particles. This will help electrons gain sufficient kinetic energy required to reach the anode. At high pressure, there are more gas molecules that would act as obstructions in the path of electrons, thereby preventing the electrons from reaching the anode.

Question 3. What was the major breakthrough in devlopment of atomic model?
Answer: J.J Thomson atomic model - all atom as a whole is electrically neutral and positively charged spreaded uniformlly through the sphere with electrons embedded in it. But Rutherford atomic model defines nucleus where all postive charge are accumulated, and electrons are present around the nucleus.

Question 4. How are cathode rays produced?
Answer: Cathode rays originate from the cathode because the cathode is a negatively charged electrode. These rays strike and ionize the gas inside the discharge tube and the electrons ejected from the ionized gas travel to the anode electrode. These rays are the stream of electrons that are actually produced from the ionized gas inside the tube.

Question 5. Was the charge and mass of electrons calculated during the cathode ray discharge tube experiment?
Answer: No, only the charge to mass ratio of electrons was calculated by the cathode ray discharge tube experiment. The charge of electrons was calculated by Millikan with an oil drop experiment. With help of these values, the mass of electrons was derived. 

Related Topics:

Millikan’s oil drop experiment

Discovery of neutrons

De Broglie's Hypothesis

Thomson's atomic model

Quantum numbers

Rutherford's Atomic Model

 

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