For a long time, atoms were believed to be the fundamental building blocks of matter, indivisible and the smallest possible particles. However, experiments conducted in the late 19th and early 20th centuries revealed that atoms were not the ultimate particles. Through persistent scientific inquiry, researchers discovered that atoms comprise even smaller constituents, leading to the identification of subatomic particles.
Subatomic particles are smaller than atoms and cannot be seen by the naked eye. The three primary subatomic particles are electrons, protons, and neutrons. Electrons carry a negative charge, protons carry a positive charge, and neutrons are electrically neutral. The four fundamental forces—gravity, electromagnetic force, strong nuclear force, and weak nuclear force—hold these particles together within the atom.
Also read: Atomic Structure
In the 19th century, British scientist William Crookes noted that gases were poor conductors of electricity. However, when the gas was placed at low pressure and subjected to high voltage, it became conductive and allowed electrical current to pass through.
J.J. Thomson, a physicist in the late 19th and early 20th centuries, made significant contributions to atomic theory. He began experimenting with cathode ray tubes, a device used to study electricity in gases.
Cathode ray tubes are sealed glass tubes with most of the air removed. When a high voltage is applied across two electrodes at opposite ends of the tube, a beam of particles, called cathode rays, flows from the negatively charged cathode to the positively charged anode. The presence of phosphors on the tube’s far end allows us to observe the cathode ray as it strikes them and causes them to glow.
Thomson placed two electric plates around the cathode ray to test its behavior. The ray was attracted to the positively charged plate and repelled by the negative plate, indicating that the cathode ray consisted of negatively charged particles.
Thomson also exposed the ray to magnetic fields and observed that it deflected, further confirming that these particles had a negative charge. By determining the mass-to-charge ratio of the particles, Thomson discovered that they were much lighter than any known atom.
Thomson's experiments revealed that the characteristics of the cathode ray were consistent regardless of the material used for the cathode, suggesting that the cathode ray was a fundamental component common to all atoms.
Observations of the Cathode Ray Experiment:
These observations confirmed that electrons are a fundamental part of all atoms.
Characteristics of Electrons:
Although Eugene Goldstein identified positively charged particles, Ernest Rutherford is credited with discovering protons.
In the late 19th century, Eugene Goldstein discovered a type of ray travelling in the opposite direction of cathode rays. Using a perforated cathode, he observed that rays emitted from the anode—now known as canal rays or anode rays—consisted of positively charged particles. Goldstein's work suggested that the rays were made up of positively charged particles.
Further analysis determined that the lightest positive particle with a unit positive charge was found when hydrogen gas was used in the discharge tube. This particle was later named the proton.
Ernest Rutherford used a glass tube filled with gas and a high-voltage source to study anode rays. The discovery of protons became clearer when these particles, after passing through a perforated cathode, were found to have a mass about 1836 times that of an electron. This led Rutherford to conclude that protons are a fundamental component of all atoms.
Observations of Anode Ray Experiment:
Rutherford's famous gold foil experiment further solidified the understanding of protons. He bombarded thin gold foil with alpha particles, and most of the particles passed through undeflected. However, a few were deflected at small angles, and some bounced back at wide angles. This suggested the presence of a tiny, dense, positively charged core at the center of the atom, which Rutherford called the nucleus. The proton was discovered to be the positively charged particle in the nucleus.
Characteristics of Protons:
James Chadwick discovered the neutron in 1932. Chadwick bombarded a thin sheet of beryllium with alpha particles and observed the emission of a neutral particle with a mass slightly greater than that of protons. These particles were called neutrons.
Characteristics of Neutrons:
(Session 2025 - 26)