Foundations of Atomic Theory: Dalton, Radioactivity, and the Electron

Dalton’s Atomic Theory

Historical Development of Atomic Theory

Dalton’s atomic theory, proposed in the early 1800s, laid the groundwork for our modern understanding of matter. It described the nature of atoms and their role in chemical reactions.

• Atoms as Fundamental Particles: Each element is composed of tiny, indestructible particles called atoms.

• Uniformity of Atoms: All atoms of a given element have the same mass and other properties that distinguish them from atoms of other elements.

• Compound Formation: Atoms combine in simple, whole-number ratios to form compounds.

• Conservation of Atoms: Atoms of one element cannot change into atoms of another element. In a chemical reaction, atoms only change the way they are bound together with other atoms.

Note: Items in blue have since been disproven by later discoveries in nuclear chemistry and physics.

Key Terms

• Atom: The smallest unit of an element that retains its chemical properties.

• Element: A substance composed of only one type of atom.

• Compound: A substance formed from two or more elements chemically combined in fixed ratios.

Example

• Water (H2O) is formed when two hydrogen atoms combine with one oxygen atom in a simple whole-number ratio.

Radioactivity

Discovery and Significance

Radioactivity is a phenomenon in which an atom breaks down, emitting particles and energy. This process provided crucial evidence that atoms are made of smaller subatomic particles.

• Marie Curie: Coined the term radioactivity and was awarded Nobel Prizes in Physics and Chemistry for her pioneering work.

• Implication: The discovery of radioactivity showed that atoms are not indivisible, as Dalton had proposed, but are composed of even smaller particles.

Key Terms

• Radioactivity: The spontaneous emission of particles or energy from unstable atomic nuclei.

• Subatomic Particles: Particles smaller than atoms, such as protons, neutrons, and electrons.

Example

• Uranium-238 undergoes radioactive decay, emitting alpha particles and transforming into other elements.

The Electron

Discovery of the Electron

The electron is a negatively charged subatomic particle. Its existence was first theorized and later demonstrated through experiments in the late 19th century.

• Faraday: Used the word electron to describe a negatively charged particle.

• J.J. Thompson (1897): Demonstrated the existence of electrons in the atom through experiments with cathode rays.

Key Terms

• Electron: A subatomic particle with a negative charge, found outside the nucleus of an atom.

• Cathode Ray: A stream of electrons observed in vacuum tubes.

Example

• Electrons are responsible for the flow of electricity in wires and for chemical bonding between atoms.

Thompson’s Experiment

Cathode Ray Tube Experiment

Thompson’s experiment with cathode ray tubes provided direct evidence for the existence of electrons.

• Setup: A partially evacuated glass tube with electrodes at each end; high voltage is applied.

• Observation: Charging the plates causes the appearance of a cathode ray, which is a beam of electrons.

• Properties of Cathode Rays:

  • Independent of the composition of the material from which they originate (the cathode).

  • Carry a negative electrical charge.

  • Have low mass.

  • Can be deflected by electric and magnetic fields.

• Conclusion: The cathode ray is made of electrons.

Key Terms

• Cathode: The negatively charged electrode in a tube.

• Anode: The positively charged electrode.

Example

• Television screens and computer monitors once used cathode ray tubes to display images.