Section 5.1: Evolution of the Atomic Model

Evolution of the Atomic Model

  • Fascination with Atoms:

    • Study of the atom has intrigued scientists for centuries, starting from Greek philosophers discussing the concept of 'atomos' as the smallest piece of matter.
    • Modern research delves into subatomic particles smaller than atoms, like neutrinos, which may unlock mysteries of the universe.

  • The Sudbury Neutrino Observatory:

    • Research on neutrinos, which have almost no mass and are difficult to detect despite being emitted in large quantities by the Sun.

  • Key Terms:

    • Atom: Smallest particle of an element retaining its identity.
    • Electron: Negatively charged subatomic particle.
    • Subatomic Particle: Particle smaller than an atom.
    • Nucleus: Central part of an atom containing protons and neutrons.
    • Proton: Positively charged particle in the nucleus.
    • Neutron: Neutral particle in the nucleus.

From Particle Theory to Atomic Theory

  • Particle Theory (Chapter 4):

    • Describes elements and compounds using particles; variations in properties of different elements and compounds.

  • John Dalton's Contribution (Early 1800s):

    • Developed Dalton's Atomic Theory after experimenting with gases and observing chemical changes:
    • Matter is made of atoms.
    • Atoms cannot be created nor destroyed.
    • Atoms of a single element are identical in mass and size, differing from those of other elements.
    • Compounds formed from atoms of different elements in definite proportions.

  • Dalton's Atomic Model:

    • Depicted as small, hard, indestructible spheres representing atoms.

  • The Size of Atoms:

    • Atoms are extremely small, approximately (10^{-10}) m in width.

Thomson's Discovery of Electrons

  • Gas Discharge Tubes:

    • Low-pressure gas tubes showed rays emitted from a negatively charged cathode towards a positively charged anode, indicating negative charges.

  • Discovery of Electrons (1897):

    • Joseph John Thomson identified rays as negatively charged particles (electrons), which are present in all atoms.

  • Thomson's Atomic Model (Raisin Bun Model):

    • Proposed atoms consist of a positively charged material with electrons dispersed throughout.
    • This was a significant advance but required further refinement.

Rutherford's Discovery of the Nucleus

  • Ernest Rutherford's Contributions:

    • Conducted gold foil experiment firing alpha particles, expecting them to pass through due to Thomson's model.
    • Instead, some particles were deflected backward, suggesting a dense positive center.

  • Rutherford's Model:

    • Proposed the Nucleus as a small, dense positively charged center of the atom, with electrons surrounding it, akin to a planetary model.

Bohr’s Description of Energy Levels

  • Bohr's Model (1912):
    • Revised Rutherford's model by proposing electrons occupy fixed energy levels around the nucleus, akin to rungs on a ladder.
    • Electrons can only absorb specific amounts of energy (quantum) to transition between these levels.

The Composition of the Nucleus

  • Protons and Neutrons:
    • The nucleus consists of protons and neutrons. Protons were identified early on, but neutrons were not discovered until 1932 by James Chadwick while building on Rutherford’s work.

Summary of Key Developments

  • John Dalton:

    • Introduced that all matter consists of atoms and defined the properties of elements.

  • J.J. Thomson:

    • Discovered the electron and proposed the raisin bun model of the atom.

  • Ernest Rutherford:

    • Established the nucleus and the planetary model of the atom.

  • Neils Bohr:

    • Introduced atomic energy levels for electron stability around the nucleus.

Review Questions

  1. What distinguishes one element from another according to Dalton?
  2. What did Thomson find about the composition of atoms?
  3. Why is Rutherford’s model referred to as the planetary model?
  4. Describe Rutherford's gold foil experiment and its implications for atomic theory.