Discovery of Sub-atomic particles
According to Dalton, an atom is an indivisible and dense sphere. Atoms of the very same elements are alike. They combine various methods to form compounds. In the light of Dalton’s atomic theory, scientists carried out a series of experiments. But in the late 1800’s and early 1900’s, researchers discovered new subatomic particles.
In 1886, Goldstein found positively charged particles called protons. In 1897, J.J. Thomson discovered in an atom, the negatively charged particles known as electrons. It was established that electrons and protons are fundamental particles of matter.
Based upon these observations Thomson put forth his “plum pudding” theory. He postulated that atoms were strong structures of positively charged with tiny negatively charged particles stuck inside. It resembles plums in the pudding.
Cathode rays and Discovery of Electron
In 1895 Sir William Crooks performed experiments by passing an electrical current through gases in a discharge tube at extremely low pressure.
He took a glass tube fitted with two metallic electrodes, which were linked to a high voltage battery. The pressure inside the tube was kept 10-4 atm. When high voltage current was gone through the gas, glossy rays were given off from the cathode which traveled towards the anode. These rays were offered the name of “cathode rays” as these were originated from the cathode.
Characteristic of Cathode rays
The cathode rays were studied in detail and their properties were determined, which are listed below:
- These rays travel in straight lines perpendicular to the cathode surface.
- They can cast a sharp shadow of an opaque object if positioned in their course.
- They are deflected towards the positive plate in an electrical field showing that they are negatively charged.
- They raise the temperature level of the body on which they fall.
- Thomson discovered their charge/mass (e/ m) ratio.
- Light is produced when these rays hit the walls of the discharge tube.
- It was found that the very same kind of rays were discharged no matter which gas and which cathode was used in the discharge tube.
All these properties suggested that the nature of cathode rays is independent of the nature of the gas present in the discharge tube or product of the cathode. The fact that they cast the shadow of an opaque item suggested that these are not rays however they are quick moving material particles.
They were offered the name electrons. Given that all the materials produce exact the same kind of particles, it implies all the materials contain electrons. As we know materials are made up of atoms, thus the electrons are fundamental particles of atoms.
Discovery of Proton
In 1886 Goldstein observed that in addition to cathode rays, other rays were also present in the discharge tube. These rays were traveling in the opposite direction to cathode rays. He used a discharge tube having a perforated cathode. He discovered that these rays traveled through holes present in the cathode and produced a radiance on the walls of the discharge tube. He called these rays as “canal rays”.
The properties of canal rays were as following:
- These rays travel in straight lines in a direction opposite to the cathode rays.
- Their deflection in the electric and magnetic fields proved that these are positively charged.
- The nature of canal rays depends upon the nature of gas, present in the discharge tube.
- These rays do not stem from the anode. In fact, these rays are produced when the cathode rays or electrons collide with the residual gas molecules present in the discharge tube and ionize them as follows:
The mass of these particles was found equal to that of a proton or easy multiple of it. The mass of a proton is 1840 times more than that of an electron.
Hence, these rays are made up of positively charged particles. The mass and charge of these particles depend upon the nature of the gas in the discharge tube. Hence, various gases produce various kinds of positively charged rays having particles of different masses. Remembering that positive particles produced by gas will be of the exact same type i.e. positive rays produced by the lightest gas hydrogen include protons.
Discovery of Neutron
Rutherford observed that the atomic mass of the element could not be explained on the basis of the masses of electron and proton only. He forecasted in 1920 that some neutral particles having a mass equal to that of protons need to exist in an atom. Thus, scientists remained in search of such a neutral particle.
Eventually, in 1932 Chadwick discovered neutron when he bombarded alpha particles on a beryllium target. He observed that highly penetrating radiations were produced. These radiations were called the neutron.
Properties of the neutron are as following:
- Neutrons carry no charge i.e. they are neutral.
- They are highly penetrating.
- Mass of these particles was almost equal to the mass of a proton.