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The Structure of the Atom_ Discovery of the Subatomic Particles

Ideas of the Ancient Greeks on the Atom and the Elements

Founders of Atomism: Leucippus and Democritus

  • Leucippus proposed that matter is made of indestructible and indivisible particles called atomos (meaning indivisible).

  • His student, Democritus, helped to explain the genesis of matter by suggesting the same.

Democritus's Experiment

  • Conducted a simple experiment with a seashell, continually breaking it until he reached a fine powder.

  • Attempted to break the smallest piece, concluding it could not be divided further, thereby defining the atom as the fundamental building block of matter.

Principles of Atomism

  • Fundamental assertions by Leucippus and Democritus include:

    • Atoms constitute all matter, separated by empty space where they move.

    • Atoms cannot fuse and instead bounce off each other upon collision.

    • Atoms are solid, homogeneous, and extremely small.

    • Variations in combinations of atoms lead to all observable changes in matter.

    • Atoms exist in different sizes, weights, and shapes, allowing for interaction, interlocking, and rebound.

    • The properties of matter reflect the properties of the atoms it contains.

Philosophical Contributions on Matter

  • Anaxagoras

    • Proposed an infinite number of basic natural substances made of near-invisible particles.

  • Empedocles

    • Suggested all matter arises from four classical elements: fire, air, water, and earth.

    • Each element corresponds to physical states of matter:

      • Fire embodies all forms of heat.

      • Air relates to gases.

      • Water encompasses all liquids.

      • Earth represents all solids.

  • Plato

    • Expanded on Empedocles' theory, introducing geometric solids known as Platonic solids, interconnected with the elements.

    • He theorized the universe was constructed from these five solids:

      • Tetrahedron corresponds to fire.

      • Cube pertains to earth.

      • Octahedron relates to air.

      • Icosahedron represents water.

      • Dodecahedron signifies the universe.

Description of Platonic Solids

  • Tetrahedron (Fire)

    • Composed of four triangular faces; associated with the element of fire due to its sharpness.

  • Cube (Earth)

    • Known as the hexahedron, features six identical square faces, distinct from various box shapes.

  • Octahedron (Air)

    • Consists of two pyramids joined at their bases; seen in natural crystal formations.

  • Icosahedron (Water)

    • Composed of 20 equilateral triangles; known for having the most faces among Platonic solids.

  • Dodecahedron (Universe)

    • Made of twelve pentagonal faces; symbolizes the cosmos, imbued with a sense of mystery.

Aristotle's Contributions

  • Proposed the four elements could balance in infinite combinations, introducing essential qualities: wet, cold, hot, and dry.

  • Suggested the balance changes affect the universe's dynamics.

The Structure of the Atom: Discovery of Subatomic Particles

  • 19th Century Developments

    • Reliance on electricity in atom studies; Michael Faraday linked electricity to atomic theory.

    • The 1890s saw a focus on radiation, leading to the invention of the cathode-ray tube (CRT).

J.J. Thomson's Discoveries

  • Conducted experiments using the CRT in 1897, discovered electrons as negatively charged particles via the mass-to-charge ratio.

  • Plum Pudding Model: Proposed a model in 1904 where electrons scatter within a positively charged 'pudding.'

Robert Millikan

  • Calculated the electron's charge; his findings helped establish its mass, earning him the Nobel Prize in Physics in 1923.

  • Contributed to the study of radioactivity alongside Marie Curie.

Ernest Rutherford's Gold Foil Experiment

  • In 1902, conducted experiments showing that atoms are mostly empty space with a dense, positively charged nucleus.

  • Proposed an atomic model similar to a solar system.

James Chadwick's Neutrons

  • Discovered neutrons in 1926, positing they have a comparable mass to protons with no charge, essential for displacing protons in the nucleus.

Niels Bohr's Atomic Model

  • Proposed electrons travel in specific orbits around the nucleus; energy quantization explains photon emission during electron transitions.

Modern Atomic Theory

  • Incorporates wave-particle duality for electrons, suggesting their positions are defined by probability within orbitals according to Heisenberg's Uncertainty Principle.

The Periodic Table

  • Dmitri Mendeleev created the first periodic table in 1869 based on atomic mass, recognizing patterns in element properties and predicting undiscovered ones.

  • Elements organized into periods (horizontal) and groups (vertical) reflecting similar properties, with Family A and B classifications.