Question

When high voltage potential difference is applied across the two electrodes in a discharge tube maintained at low-pressure $({10}^{-6}\mathrm{atm})$ certain rays were produced from the surface of the cathode. They were called cathode rays. These gases ionize the gas through which they pass. The positive ions so produced move in the opposite direction and are called anode rays. The study of the properties of these rays led to the discovery of two important sub-atomic particles. Read the passage and answer the following questions.

1. Which sub-atomic particles was discovered by the study of cathode rays. (a) Protons (b) positrons (c) electrons (d) neutrons
2. What is the value of the elm (charge/mass) ratio of cathode rays?
3. Which fundamental sub-atomic particle was discovered by the detailed study of positive rays?
4. What is the mass of electrons on the atomic mass scale?

Answer 1

(i)

The explanation for the correct option -

• The cathode rays are the negatively charged rays produced from the surface of the cathode when a high voltage potential difference is applied across the two electrodes in a discharge tube.
• These rays are made up of electrons, which are tiny, negatively charged particles.
• Because all gases are produced by cathode rays, it was determined that all atoms contain negatively charged particles known as electrons.

The explanation for the incorrect options -

• Option A is incorrect because protons are formed by the study of anode rays.
• Option B is incorrect positrons is a subatomic particles with the same mass as an electron and a numerically equal but positive charge. These particles are formed from the decay of nuclides.
• Option D is incorrect because neutrons are the neutral particles formed by the bombardment of alpha particles on the sheet of Beryllium.

Hence, option C is the correct answer.

(ii)

• Cathode rays are made up of electrons, which are tiny, negatively charged particles.
• Because all gases produce cathode rays, it was determined that all atoms contain negatively charged electrons independent of the nature of the gas.
• The charge of an electron is $-1.6\times {10}^{–19}\mathrm{C}$ and the mass of an electron is$9.1\times {10}^{-31}\mathrm{Kg}$.
• The (charge/mass) ratio for cathode rays = $-1.76\times {10}^{11}\mathrm{C}{\mathrm{Kg}}^{-1}$

(iii)

• When a high voltage potential difference is applied across the two electrodes in a discharge tube, the anode produced streams of heavy particles (positive electrode). Anode rays are the name for these particle streams.
• These rays are made up of protons

(iv)

• The mass of electrons on the atomic mass scale is $\frac{1}{1840}$ of the mass of a hydrogen atom.
• The mass of one atom of hydrogen in grams is $1.66058\times {10}^{-24}\mathrm{g}$. Hence, the mass of an electron in grams is $9.1\times {10}^{-28}\mathrm{g}$