Discovery of Subatomic Particles

1.0Introduction

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

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

2.0The Discovery of Electrons

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.

The Cathode Ray Experiment

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:

1. Cathode rays originate from the cathode and move toward the anode.
2. They are visible only when they strike phosphorescent materials, creating a greenish-yellow glow.
3. In an electric field, the rays are deflected toward the positive plate and repelled by the negative plate, moving in straight lines without external forces.
4. Cathode rays can produce X-rays when they strike hard metals.
5. These rays can penetrate matter.

These observations confirmed that electrons are a fundamental part of all atoms.

Characteristics of Electrons:

• The mass of an electron is approximately 1/1836 the mass of a hydrogen atom.
• An electron carries a unit negative charge with a magnitude of 1.6 x 10^-19 coulombs.
• The electron is incredibly small, with a radius smaller than 10-¹⁵ meters.

3.0The Discovery of Protons

Although Eugene Goldstein identified positively charged particles, Ernest Rutherford is credited with discovering protons.

Goldstein's Experiment

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.

Rutherford's Experiment to Discover Protons

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:

1. Anode rays travel in straight lines.
2. They originate at the anode and travel toward the cathode.
3. In an electric field, anode rays are attracted to the negative plate and repelled by the positive plate.
4. Anode rays produce a heating effect when they strike a metallic foil.
5. The particles in anode rays are microscopic and cause mechanical effects.

Rutherford’s Gold Foil 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:

• A proton’s mass is similar to that of a hydrogen atom, roughly 1836 times the mass of an electron.
• A proton carries a positive unit charge of 1.6 x 10^-19 coulombs.
• Protons are located in the nucleus of the atom.

4.0The Discovery of Neutrons

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:

• Neutrons are electrically neutral and are unaffected by electric or magnetic fields.
• The mass of a neutron is slightly greater than that of a proton.

5.0Difference Between Protons, Electrons, and Neutrons