In this post, we will discuss about the anode rays and the discovery of proton and neutron.
Anode Rays (Positive Rays or Canal Rays)
During his working with cathode rays, Sir J J Thomson had noticed a red glow around the cathode, but on the side opposite the anode. To know the reason for this glow, he designed a discharge tube with a perforated cathode fixed in the middle of the tube.
On applying electric potential across the two electrodes, a green fluorescence was seen in the glass tube at one end, while on the other end, a red glow was seen. The green glow, called fluorescence, was due to the bombardment of glass by the cathode rays. Obviously, the red glow was due to some other type of emission.
How are anode rays produced?
Cathode rays consist of negatively charged particles called electrons. These electrons move away from the cathode with very high speeds. These fast-moving electrons split the gas molecule into atoms, and remove one or more electrons from the atom. Thus, the atom get converted into positive ion due to the loss of electrons. These positive ions pass through the holes in the cathode plate to produce a glow in the glass wall of the discharge tube.
A stream of these positively charged particle is called positive ray or anode ray.
What are the properties of anode rays?
- Positive rays consist of positively charged particles.
- The nature of these rays depends on the gas used in the discharge tube.
- These rays travel in straight lines.
- These rays get deflected by an electrical field, and bend towards the negative plate. Thus, the deflection of the positive rays is in a direction opposite to that shown by the cathode rays.
- These rays are also deflected by the magnetic field in a direction opposite to that of cathode rays.
- These rays can produce mechanical as well as chemical effects.
- The ratio of charge to mass, i.e., e/m, for the particles in the positive rays is not the same for all gases.
- The ratio e/m for the positive rays is very low as compared to the e/m value for cathode rays.
This shows that the particles in the positive rays are heavier than the particles in the cathode rays.
The Proton
How was proton discovered?
Goldstein, in 1886, found that the charge-to-mass (e/m) ratio of the positive ray particles depend upon the nature of the gas in the discharge tube, i.e., e/m values for the positive rays of different gases are different. The e/m value for the positive rays was the highest when hydrogen gas was used in the discharge tube. This is because hydrogen atom is the lightest atom of all, i.e., m is the lowest and hence, e/m is the highest.
The particle in the positive rays when hydrogen gas was used in the discharge tube, was given the name proton. A proton is produced when an electron is removed from hydrogen atom.
H → H+ + e–
Thus, the proton is a hydrogen ion (H+).
What are the characteristics of a proton?
Proton is positively charged particle. Its physical characteristics are:
- Mass of a proton: The mass of a proton is equal to that of hydrogen atom. Its mass is about 1840 times that of an electron. Mass of proton = 1.676 * 10-27 Kg = 1.676 * 10-24 g
- Charge on a proton: A proton carries a charge equal in magnitude, but opposite in sign to that carried by an electron. Thus, it has a unit positive charge. Charge on proton = +1.602 * 10-19 C
The Neutron
How was neutron discovered?
In 1920, Rutherford found that except hydrogen atom, the atomic masses of other atoms could not be explained on the basis of only electrons and protons, e.g., helium atom contain two protons. As a result, the atomic mass of helium should be double that of hydrogen atom. From the e/m values, it was discovered that the mass of helium atom (to be precise that of helium ion), is four times the mass of a hydrogen atom. To explain this, Rutherford, in 1920, predicted the presence of another kind of particle in atoms. He further predicted that such a particle should be electrically neutral and have a mass equal to that of proton.
In 1932, James Chadwick discovered neutron.
What are the characteristics of a neutron?
- Mass of a neutron: The mass of neutron is equal to that of proton. Mass of a neutron = 1.676 * 10-27 Kg = 1.676 * 10-24 g
- Charge on neutron: Neutron does not carry an electric charge. So neutrons are neutral particles.
How do electrons protons neutrons compared with each other?
Electrons, protons and neutrons are the most important subatomic particles for chemists. Their properties are compared in the table given below:
| Property | Electron | Proton | Neutron |
| Mass | 9.1*10-31 Kg or 9.1*10-28 g | 1.676*10-27 Kg or 1.676*10-24 g | 1.676*10-27 Kg or 1.676*10-24 g |
| Charge | -1.602*10-19 C | +1.602*10-19 C | Zero |
Read More: Atoms and Molecules
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