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Every matter is made up of some small particles called atoms. These atoms can neither be created nor be destroyed. Atoms were considered as the smallest unit of a particle until the Discovery of Electrons.
The topic ‘Discovery of Electron’ is a part of Unit 2 ‘Structure of Atom’. It is among one of the most important topics of CBSE Class 11 Chemistry. Both Unit 1 and Unit 2 carry a combined weightage of 11 Marks in the CBSE Class 11 Examination.
Read Also: Dalton’s Atomic Theory
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Keyterms: Atoms, Electron, Nucleus, discharge tube, protons, neutrons, energy levels, magnetic field
What are Electrons?
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Electrons are subatomic particles that are negatively charged and revolve around the nucleus of a cell. JJ Thomson discovered the electrons based on his experiments performed in the discharge tube. This discovery proved that atoms have a complex structure.
Electrons
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Characteristics of Electron
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- Electrons are negatively charged particles that revolve around the nucleus which is at the centre of an atom consisting of the proton and neutron.
- Unlike protons or neutrons, electrons are the only fundamental particles of an atom that cannot be further divided. They are represented by e or e-.
- Electrons have different energy levels and can be moved to higher energy levels by absorbing some amount of energy.
- Electrons do not contribute to the overall mass of atoms as they have an insignificant mass of 9.1094 x 10-³¹ kg, which is about 1/2000 the mass of a proton or neutron.
- The electric charge of an electron is -1 which is opposite to the electric charge of a proton which is +1.
- The force of attraction between negatively charged electrons and positively charged protons helps electrons move in a particular orbit around the nucleus.
Discovery of Cathode rays
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In 1878, William Crooks, a British physicist, first conducted experiments on discharge tubes which were evacuated sealed tubes. When high voltage was applied to a gas at low pressure in the discharge tube, it resulted in the flow of electricity through the gas thereby emitting fluorescence at the other end of the discharge tube.
This was due to the movement of charge from a negative plate called the cathode to a positive plate called the anode. The rays were termed cathode rays as they were emerging from the cathode plate.
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JJ Thomson’s experiment and the discovery of electrons
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In 1897, Joseph John Thomson, a British physicist proved that atoms are not the fundamental unit of matter. Thomson began experimenting with cathode ray discharge tubes and studied the properties of the cathode ray.
In his experiment, he placed two plates of opposite charges near the cathode rays. After applying high voltage, the cathode rays were deflected from the negatively charged plate and moved towards the positive plate. Since like charges repel each other and opposite charges attract each other, this hinted that cathode rays consisted of negatively charged particles which are now known as electrons.
Thomson then placed two magnets on either side of the plates to study the behavior of cathode rays in the presence of a magnetic field. His observation was that the presence of a magnetic field also deflected the cathode rays. The extent of deflection helped him determine the mass to charge ratio. The mass of cathode ray particles was much smaller than the mass of the atom.
E/m= -1.76x 108 coulomb per gram
Thomson repeated the experiment by changing electrode materials but the results remained the same which led to the belief that cathode rays are the same properties no matter which cathode material did they originate from or which gas was used in the set up thus postulating that these particles were the fundamental unit of all atoms.
Also Read:
| Related Articles | ||
|---|---|---|
| Electron Emission | Classification of Matter | Bond Enthalpy |
| De Broglie Relationship Important Notes | Periodic Classification of Elements | First 20 Elements |
The conclusions drawn by Thomson through his experiments were as follows:
- Cathode rays are composed of negatively charged particles and they travel in a straight line.
- The mass of each particle was only equivalent to 1/2000 the mass of a hydrogen atom.
- These negatively charged particles are found in every atom.
Some Important Points
- Thomson’s experiments were recognized and widely accepted and his cathode ray particles were known as electrons. As a result of Thomson’s experiments, a new model of the atom was later developed to understand the structure of an atom.
- The charge of the electron was determined by an American scientist named Robert Millikan. He made use of electrically charged oil droplets to calculate the charge on a single electron. According to him, the charge of a single electron was 1.6 x 10-19 Coulomb.
- With Thomson’s mass to charge ratio and charge on a single electron, Millikan calculated the mass of an electron. The mass of the electron is 9.1094 x 10-³¹ kg.
- Thomson was awarded the Nobel prize in 1906 for discovering the fundamental particle electron.
Previous Year Questions
- The momentum of a particle having a de-Broglie wavelength of 1017 m is (Given, h = 6.625×10−34 m)...[NEET 1996]
- The orbitals are called degenerate when:….[NEET 1996]
- Calculate the energy in joule corresponding to light of wavelength 45 nm...[NEET 2014]
- The energy of second Bohr orbit of the hydrogen atom is −328kJmol−1, hence the energy of fourth Bohr orbit would be...[NEET 1980]
- According to the Bohr Theory, which of the following transitions in the hydrogen atom will give rise to the least energetic photon?..[NEET 2011]
- 4d, 5p, 5f and 6p orbitals are arranged in the order of decreasing energy. The correct option is :..[NEET 2019]
- Uncertainty in position of an electron (mass of an electron is =9.1×10−28g)….[NEET 1995]
- Electronic configuration of calcium atom can be written as..[NEET 1992]
- If the energies of the two photons are in the ratio of 3 : 2, their wavelengths will be in the ratio of...[KCET 2011]
- Among the elements from atomic number 11 to 36, the number of elements which have an unpaired electron in their s subshell is...[KCET 2014]
- Uncertainty principle is valid for...[KEAM]
- The energy of an electron in the 3S orbital (excited state) of H - atom is...[KEAM]
- The line of longest wavelength corresponds to n2=3...[KEAM]
- Which of the following is the correct unit of angular momentum of an lectron in an orbital of an atom?..[KEAM]
- What is the wavelength (in mm ) of a particle of mass 6.62×10−29g moving with a velocity of 103ms−1?...[JIPMER 2009]
Important Questions
Ques.1: Which experiment led to the discovery of electrons and how? (1 Marks)
Answer: The cathode ray discharge tube experiment performed by J.J. Thomson led to the discovery of negatively charged particles called electrons.
The cathode ray discharge tube consists of two thin pieces of metals called electrodes sealed inside a glass tube with sealed ends. The glass tube is attached to a vacuum pump and the pressure inside the tube is reduced to 0.01mm. When fairly high voltage (10, 000V) is applied across the electrodes, invisible rays are emitted from the cathode, called cathode rays. These rays led to the discovery of electrons.
Ques.2: (a) Name the subatomic particles of an atom. (1 Marks)
(b)What is the charge (e) of an electron?
(c)What is the e/m ratio of an electron?
Answer: (a) The subatomic particles of an atom are electrons, protons and neutrons.
(b) The charge of an electron is 1.6 x 10-19 Coulomb.
(c) The e/m ratio of an electron is -1.76x 108 coulomb per gram
Ques.3: What was Thomson’s idea of atomic structure? Why was Thomson’s atomic model rejected? (1 Marks)
Answer: Thomson after discovering the presence of negatively charged particles suggested the plum pudding model of the atom in which he suggested the atom to be a sphere of positive charge and negatively charged electrons are fixed into it to balance out the total charge. The electrons are like plums in a pudding and hence it is commonly known as the plum pudding model.
Thomson’s atomic model was rejected as it could not explain the distribution of electrons in the atom. His postulation that the total mass of an atom was uniformly distributed was not in sync with the results of his experiments.
Ques.4: How was the charge on one electron calculated? (1 Marks)
Answer: Robert Millikan calculated the charge on a single electron through his oil-drop experiment. It is represented by e and calculated in coulombs. The charge of an electron is always an integral value of e=1.6 x 10-19 C.
Ques. 5: (a) Why do electrons move in a particular orbit? (1 Marks)
(b) Who named electrons?
Answer: (a) Electrons move around the nucleus in a particular orbit due to the attraction caused by positively charged protons. Since opposite charges attract each other, protons keep the electrons moving around in an orbit.
(b) In 1891, George Johnstone Stoney named unit charge as electrons in his experiments that passed an electric current through different chemicals.
Ques.6: What is a nucleus? (1 Marks)
Answer: The nucleus is a dense but small area at the centre of an atom composed of positively charged particles called protons and electrically neutral particles called neutrons. Ernest Rutherford is credited with the discovery of the nucleus.
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