Atomic Structure and Subatomic Particles

The Concept of Matter and its Structural Units

All matter is composed of structural units called atoms. This concept can be illustrated using the analogy of a building, which is composed of many flats and rooms. Every room consists of four walls, and these walls are constructed from bricks. In this context, the brick is the fundamental structural unit of the building. In a similar manner, atoms function as the structural units of all matter in the universe. Various chemical substances display distinct properties based on their composition. For instance, when a piece of sodium metal is introduced into a beaker of water, it reacts violently and burns. An iron nail exposed to open air undergoes a slow reaction with oxygen and moisture, gradually turning reddish brown. Water, often referred to as the elixir of life, possesses properties entirely different from its constituent elements, hydrogen and oxygen. Elements like sodium and iron are composed of the same kind of atoms, whereas compounds like water are formed by the chemical combination of different kinds of atoms.

Historical Perspectives on Atomism

The idea that matter is divisible is an ancient concept. In India, the philosopher Maharishi Kanada, around $600\,B.C.$, proposed that if one continues to divide matter, the particles will become smaller and smaller. Eventually, a point is reached where further division is impossible. Kanada named these ultimate, indivisible particles paramanu. In the $5^{th}$ century B.C., the Greek philosopher Democritus presented a similar concept, asserting that all matter consists of small, indivisible particles. He called these particles atoms, a term derived from the Greek word 'atomos,' which literally means indivisible. While these philosophical ideas existed for centuries, the scientific concept of the atom only resurfaced around the $18^{th}$ century when John Dalton proposed a formal theory to explain the behavior of chemicals.

Dalton's Atomic Theory (1803)

John Dalton proposed a set of postulates to explain the nature of atoms and their role in chemical reactions. First, all matter is composed of tiny, indivisible particles known as atoms. Second, atoms are permanent entities that can neither be created nor destroyed during chemical reactions. Third, all atoms of a specific given element are identical in terms of their mass and their chemical properties. Fourth, atoms of different elements possess different masses and different chemical properties. Finally, atoms of different elements combine in ratios of small whole numbers to form chemical compounds. This theory provided an essential insight into why different chemical substances exist, though later experiments regarding electricity raised questions about the supposed indivisibility of the atom.

The Cathode Ray Experiment (1897)

Under ordinary conditions, gases do not conduct electricity. However, they become conductive when subjected to high voltage at very low pressures. J. J. Thomson utilized a discharge tube, an apparatus invented by William Crookes known as the Crookes' tube, to study this phenomenon. The discharge tube is constructed of hard glass (quartz) and contains two thin metal pieces called electrodes. The positive electrode is the anode, and the negative electrode is the cathode. At a normal pressure of $1\,atm$, no conduction occurs. Thomson applied a voltage of $10,000\,V$ and evacuated the tube to a very low pressure of $0.001\,mm Hg$. For context, $1\,atm$ is equivalent to $760\,mm Hg$.

To detect the discharge, a fluorescent material, zinc sulphide ($ZnS$), was coated on the inner side of the discharge tube at the anode end. When the high voltage was applied, current flowed, and flashes of light appeared on the zinc sulphide screen behind the anode. This confirmed the presence of particles moving through the tube. By using a perforated anode and applying an external electric field, it was observed that these rays deflected toward the positively charged plate, indicating they carried a negative charge.

Properties of Cathode Rays and the Electron

Cathode rays exhibit several distinct physical properties. They travel in straight lines, as evidenced by the sharp shadow cast by an object placed in their path. They possess enough energy to penetrate thin metallic foils, and when they are focused on such foils, the metal heats up. These rays consist of material particles; this was demonstrated by placing a light paddle wheel in their path, which began to rotate upon impact. Furthermore, cathode rays consist of negatively charged particles that deflect toward a positive plate in an electric field. When these rays strike certain metals like copper or tungsten, they produce X-rays.

Critically, the ratio of charge to mass ($e/m$) for cathode rays remained constant regardless of the gas used in the tube or the material of the electrodes. This consistency led to the conclusion that cathode rays are composed of the same fundamental particles, which were named electrons. The term 'electron' was coined by the Irish physicist G. J. Stoney. Electrons are considered universal particles found in all matter. The charge-to-mass ratio for an electron is calculated as follows:

$\frac{e}{m} = \frac{1.6 \times 10^{-19}\,C}{9.1 \times 10^{-31}\,kg} = 1.758 \times 10^{11}\,C/kg$

The Canal Ray Experiment (1886)

Because atoms are electrically neutral and electrons are negatively charged, scientists reasoned that atoms must also contain positively charged particles. Eugen Goldstein discovered these positive particles using a modified discharge tube equipped with a perforated cathode. When high voltage was applied at low pressure, rays were observed moving in the direction opposite to cathode rays. These rays, which passed through the holes or "canals" in the cathode, were termed canal rays. They were found to be composed of positively charged ions created from the gas within the tube.

Properties of Canal Rays

Canal rays travel in straight lines, similar to cathode rays, and are composed of material particles. However, they originate from the gas atoms within the discharge tube rather than the cathode itself. Compared to cathode rays, canal rays have less penetrating power. The particles within these rays carry a positive charge. When hydrogen gas was used in the discharge tube, the resulting particles had a mass almost equal to that of a hydrogen atom and carried a charge equal in magnitude but opposite in sign to the electron. This specific particle was identified as the proton. The mass of these particles is generally observed to be multiples of the mass of the hydrogen ion.