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.