Everything around us is made up of tiny particles known as atoms. The existence of atoms has been proposed since the time of early Indian and Greek philosophers. According to these philosophers, if a piece of matter is continuously subdivided into smaller pieces, there eventually comes a point where it can no longer be divided further. This final, non-divisible stage of matter is known as the atom. The word “atom” is derived from the Greek word atomos, which means “uncuttable” or “non-divisible.” Atoms are extremely small particles, and everything around us is composed of the repeated arrangement of atoms. Despite their fundamental role in matter, scientists have long been puzzled by several questions: What do atoms look like? How do atoms of different substances differ from one another? And, perhaps most intriguingly, is the atom truly indivisible?
Early Theories of the Atom
The first description of an atom was given in 400 BC by the Greek philosopher Democritus. According to him, atoms are small, hard, incompressible particles made of a single material and existing in different shapes and sizes. Democritus believed that the different physical and chemical properties of matter were due to the different shapes and sizes of atoms. He theorized that atoms are in constant motion, and when they collide, they stick to each other. This concept laid the foundation for the modern understanding of the atom, though it remained largely speculative for centuries.
John Dalton’s Atomic Theory
In 1808, the ideas of Democritus were enhanced by the English chemist and physicist John Dalton, who introduced the first atomic theory. Dalton’s model of the atom is often referred to as the billiard ball model, as he believed that atoms were extremely small, hard, indivisible particles, much like billiard balls. Dalton’s theory was based on several key postulates:
- All atoms of a particular element are similar in all respects, including their physical and chemical properties.
- Atoms of different elements have different physical and chemical properties.
- Atoms combine in whole-number ratios to form stable compounds.
While Dalton’s atomic theory was a significant advancement, it was later discovered to have limitations. For instance, the existence of isotopes—atoms of the same element with different masses—proved Dalton’s first postulate wrong. Isotopes arise due to differences in the number of neutrons, which Dalton did not account for. Additionally, the discovery of isobars, atoms of different elements with the same number of nucleons, contradicted Dalton’s second postulate. Despite these limitations, Dalton’s work remains a landmark in the history of atomic theory.
The Discovery of Subatomic Particles
For centuries, atoms were assumed to be the smallest indivisible particles. However, this changed as new discoveries were made. In 1854, Heinrich Jaesler, a glassblower working with German physicist Julius Plucker, was experimenting with vacuum tubes. In 1858, Plucker enclosed two electrodes within a vacuum tube and applied a high voltage, causing an electric current to flow between them. This experiment revealed a mysterious green glow on the inner surface of the glass tube. Initially, the glow was thought to be a property of the electric current, but further investigation revealed that the glow was produced by rays emanating from the cathode.
In 1869, Julius Plucker and Wilhelm Hittorf repeated the experiment with improved vacuum tubes. They observed that the glow produced in the tube was independent of both the gas inside and the material of the electrodes. When an object was placed in front of the cathode, it cast a shadow, confirming that the glow was caused by rays emitted from the cathode. These rays became known as cathode rays.
The Nature of Cathode Rays
The nature of cathode rays was controversial. Most French and British physicists believed they consisted of electrically charged particles, while many German physicists considered them to be some form of waves. In 1879, English physicist and chemist William Crookes investigated cathode rays and found that they were deflected by a magnetic field. This suggested that cathode rays were composed of negatively charged particles. When high voltage was applied to electrodes in a Crookes tube, the charged particles from the cathode collided with a paddle wheel, causing it to move. This demonstrated that cathode rays carried momentum and confirmed their particle nature.
Discovery of the Electron
In 1897, English physicist J.J. Thomson conducted further experiments on cathode rays, exploring their behavior in electric and magnetic fields. Through these experiments, Thomson not only confirmed the particle nature of cathode rays but also determined the ratio of charge to mass of these particles, based on their deflection in the fields. He named these particles corpuscles, which were later identified as electrons. This discovery revealed that the atom is not indivisible, as previously believed, but instead contains subatomic particles, with the electron being the first to be identified.
The Plum Pudding Model
The discovery of the electron raised new questions about how electrons are arranged in an atom. In 1904, J.J. Thomson proposed the first atomic model to account for the subatomic particles. This model, known as the plum pudding model, suggested that electrons are distributed uniformly within a positively charged sphere of the atom, similar to plums scattered in a pudding. Though this model would eventually be replaced by more accurate models, it was a crucial step in the development of atomic theory.
