Dalton's Atomic Theory, Subatomic Particles, and Rutherford's Gold Foil Experiment

Dalton's Atomic Theory and Combination of Elements

• Elemental Combinations and Compound Formation:

  • Elements combine in specific numerical ratios to form distinct chemical compounds.
  • Hydrogen ($H$) and Oxygen ($O$) combine in a $2:1$ ratio to form water ($H_2O$).
  • Hydrogen ($H$) and Oxygen ($O$) combine in a $2:2$ ratio to form hydrogen peroxide ($H_2O_2$).
  • Water ($H_2O$) and hydrogen peroxide ($H_2O_2$) are the only two stable compounds composed exclusively of hydrogen and oxygen.
  • When Carbon ($C$) combines with Hydrogen ($H$) and Oxygen ($O$), the number of possible resulting chemical compounds becomes exceedingly large.

• Postulates of Dalton's Atomic Theory:

  • Postulate 1: Fundamental Particles: Every element is composed of extremely small, indivisible particles called atoms, representing the smallest unit of matter that cannot be divided further.
  • Postulate 2: Identical Properties of Atoms: All atoms of a given element are identical to one another in mass and all other chemical and physical properties.
  • A carbon atom in any location possesses the exact same mass as any other carbon atom in the universe.
  • A lithium atom in any location possesses the exact same mass and chemical reactivity as any other lithium atom in the universe.
  • Postulate 3: Compound Formation in Small Whole-Number Ratios: Chemical compounds form when atoms of different elements combine or fuse together in simple, whole-number ratios.
  • Compounds exist in precise fixed ratios (e.g., water is composed of an exact $2:1$ ratio of hydrogen to oxygen, rather than fractional ratios like $1.5:3$).
  • Postulate 4: Conservation and Rearrangement of Atoms: Atoms of an element are not transformed into atoms of a different element by chemical reactions; chemical reactions consist solely of the rearrangement of existing atoms.

Chemical Reactions and Atomic Rearrangement

• Conservation of Elements in Reactions:

  • Historical theories posited that elements transformed into one another (e.g., converting water into air through the addition of elemental fire, water, or air).
  • Atomic theory establishes that elements do not convert into other elements during chemical processes.
  • In the electrolysis of water ($H_2O$), electrical energy breaks the chemical bonds between oxygen and hydrogen atoms. The individual atoms are rearranged into new bonds, yielding oxygen gas ($O_2$) and hydrogen gas ($H_2$).

• Elemental Analysis of Combustion (Campfire Example):

  • Reactants:
  • Wood: Composed primarily of Carbon ($C$), Hydrogen ($H$), Nitrogen ($N$), and Oxygen ($O$).
  • Atmospheric Oxygen ($O_2$): Required for the combustion process to proceed.
  • Products:
  • Charcoal: Residual elemental Carbon ($C$).
  • Carbon Dioxide ($CO_2$): Composed of Carbon and Oxygen.
  • Water ($H_2O$): Produced as a standard byproduct in all hydrocarbon combustion processes.
  • Smoke and Heat energy.
  • Elemental Balance: All elements present in the starting materials (reactants) match the elements present in the resulting products.

Discovery and Properties of the Electron

• Cathode Ray Tube Experiments:

  • Developed to determine internal atomic structure and explain patterns in atomic behavior.
  • Operates on principles similar to fluorescent lighting and modern neon lights.
  • Led directly to the discovery of the electron in $1897$.

• Experimental Setup and Deflection:

  • Apparatus: A sealed glass tube containing two metal electrodes connected to a high-voltage battery terminal, producing a negatively charged electrode (cathode) and a positively charged electrode (anode).
  • Beam Formation: High voltage drives a stream of electrons traveling continuously through the tube from the negative terminal toward the positive terminal.
  • Electromagnetic Field Deflection: When exposed to an external electric or magnetic field with a positively charged top plate and a negatively charged bottom plate, the electron beam is perturbed and deflects toward the positive plate.

• Properties of the Electron:

  • Electrons carry a negative electrical charge.
  • By adjusting the electromagnetic field strength and evaluating particle velocity and beam curvature, calculations established that electrons are approximately $2000$ times lighter than a proton ($\text{Mass}_{electron} \approx \frac{1}{2000} \times \text{Mass}_{proton}$).
  • Revision of Atomic Theory: The existence of subatomic electrons disproved the postulate that atoms are indivisible units of matter.

Rutherford's Gold Foil Experiment and the Atomic Nucleus

• Experimental Setup:

  • Designed to test whether matter is solid or composed primarily of empty space.
  • Target: Extremely thin gold foil.
  • Radiation Source: A radioactive rock undergoing atomic decay, housed inside a lead-lined box equipped with a small pinhole opening to project a focused beam of high-energy particles.
  • Detection: High-energy particles were measured using particle detectors equivalent to Geiger counters.

• Alpha Particle Characteristics:

  • The high-energy particles emitted by the radioactive decay source were alpha particles ($\alpha$ particles).
  • Each alpha particle consists of $2$ protons and $2$ neutrons.
  • Alpha particles are identical to doubly ionized helium nuclei ($He^{2+}$), carrying a net electrical charge of $+2$.

• Experimental Results and Structure of Matter:

  • Observation: The vast majority of alpha particles passed directly through the gold foil unhindered; only an extremely small fraction experienced deflection or bounced back.
  • Conclusion: Atomic matter consists almost entirely of empty space, despite macroscopic objects appearing completely solid.