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Structure Of An Atom: Discovery of Subatomic Particles.

Updated on 07 December 2021
NEET Preparation
23 min read 2 views
Updated on 07 December 2021

Definition and types of subatomic particles

A particle smaller in size than an atom is referred to as a subatomic particle. The subatomic particles of an atom include: electrons, protons, and neutrons.

Earlier, the thought that an atom is the final and endmost particle that cannot be divided into smaller parts, was commonly prevalent. However, the discovery of subatomic particles through experiments conducted by scientists in the nineteenth and twentieth centuries led to a belief that atoms can be divided into subatomic particles, which are protons, neutrons and electrons.

Subatomic particles and their discovery

Electron and its discovery

William Crooks conducted discharge tube experiments in 1878 and discovered new radiations, which he named cathode rays. They were named so because the rays were traveling from the cathode to the anode. Later, J.J Thomson investigated the properties of cathode rays and concluded that they are negatively charged particles known as electrons. 

The cathode rays have the following characteristics:

1. Cathode rays follow a straight-line path. As a result, cathode rays throw a shadow of a solid object that comes in their way. The positioning of the anode does not affect the path traveled by the cathode rays.

2. Cathode rays are made up of matter particles and contain energy as a consequence of their velocity and mass. Cathode rays can move a paddle wheel placed in their path.

3. Cathode rays contain negatively charged particles. When placed in an electric field, they get deviated towards the positively charged anode plate.

4. Cathode rays can heat an object. The particles of cathode rays have kinetic energy. On colliding with an object, they transfer some of their kinetic energy to that object. This further leads to an increase in the temperature of that object.

5. Glass surface experiences a green fluorescence when cathode rays fall on it.

6. Thin metallic sheets can be penetrated by cathode rays.

7. Cathode rays can ionize the gases that fall in their path.


Electrons and their properties:

Electrons are the type of subatomic particles that orbit around an atom's nucleus. Loss or gain of electrons result in formation of an ion. Chemical bonding occurs when electrons from different atoms bind together. The properties of the electrons are described below:

  1. Electrons are subatomic particles with a negative charge.
  2. All elements have an equal number of electrons and protons in their atoms.
  3. When compared to the mass of a proton, the mass of an electron is insignificant. It is discovered to have a mass of (1/1837) times that of a proton.
  4. -1.602 * 10-19 Coulombs is the charge of an electron.



During a gold foil experiment in 1911, renowned scientist Ernest Rutherford discovered the proton. Rutherford used an ultrathin gold foil to bombard Alpha particles, which he discovered scattering on a zinc sulfate screen. The following were the experiment's observations:

1. The majority of the alpha particles fired upon the foil went through it without being deflected.

2. A tiny number of alpha particles were deflected at an angle.

3. Only a few alpha particles bounced off the foil's surface.

The following conclusions were formed based on the observations:

1. The majority of the rays passed through the foil undeflected, indicating that the atom has a huge number of unoccupied spaces.

2. The presence of a few deflected particles suggests that the atom contains some oppositely charged particles that cause the deflection while maintaining electrical neutrality.

3. Finally, the small proportion that bounced back demonstrates that a small component of the atom has a positive charge concentrated in it. The 'nucleus' is the name given to this concentrated charged center.

Rutherford's Gold Foil experiment provided valuable insights into the structure of an atom. 

Properties of a proton

  1. Protons are positively charged subatomic particles that make up an atom's nucleus. 
  2. The elements in a periodic table are organized in increasing order of atomic number. The atomic number of an element is equal to the number of protons in that element's atom. 
  3. The proton is denoted by the letter p.
  4. Because protons are about 1800 times heavier than electrons, they have an impact on an element's atomic mass.


Neutrons and their properties:

  1. The neutrally charged subatomic particles found in the nucleus are called neutrons. They are present along with positively charged protons inside the nucleus.
  2. The mass of the neutrons is 1.67 *10 -24 times that of a proton.
  3. The atomic mass of an element is equal to the sum of its neutrons and protons. Neutrons + Protons = Atomic mass number (A). (An atomic mass number is a number that represents the mass of an atom.)
  4. A neutron has a density of 1.5*10 14 g/cc.

In 1935, James Chadwick was given the Nobel Prize for his contributions to science through the discovery of neutrons. Ernest Rutherford, on the other hand, was the first to conceptualize the term "neutron." Chadwick carried out the following experiment:

  1. Chadwick attacked a beryllium sheet with alpha particles produced by the radioactive disintegration of polonium.
  2. Uncharged radiation was produced as a result of the bombardment.
  3. The surface of a paraffin sheet was allowed to interact with these uncharged penetrating radiations.
  4. Positively charged hydrogen atoms are abundant in paraffin sheets. The Geiger Counter identified hydrogen atoms being emitted from the paraffin layer after being bombarded with penetrating radiations.

Chadwick's experiment yielded the following conclusions:

The uncharged particles that were ejected had the same mass as protons, based on the displacement of protons from the paraffin sheet.

Neutrons is the new name for these uncharged particles.

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