Foundations of Atomic Theory: Early Discoveries in Chemistry

Dalton’s Atomic Theory

Historical Foundations of Atomic Structure

Dalton’s atomic theory, proposed in the early 1800s, laid the groundwork for our understanding of matter at the atomic level. Although some aspects have since been disproven, the theory remains a cornerstone of modern chemistry.

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

• Identical 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 chemical reactions, atoms only change the way they are bound together with other atoms.

Note: Items in blue have since been disproven by later discoveries.

Radioactivity

Discovery and Significance

Radioactivity is a phenomenon in which an atom breaks down, providing evidence that atoms are made of smaller particles. This discovery was crucial in disproving the idea that atoms are indivisible.

• Marie Curie: The term radioactivity was coined by Marie Curie, who won Nobel Prizes in both Physics and Chemistry for her pioneering work.

• Implication: Radioactivity demonstrated that atoms could transform and emit subatomic particles, leading to the identification of electrons, protons, and neutrons.

The Electron

Identification of Subatomic Particles

The electron was the first subatomic particle to be discovered, fundamentally changing our understanding of atomic structure.

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

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

Thomson’s Experiment

Cathode Ray Tube and the Discovery of the Electron

Thomson’s experiment involved passing electricity through a partially evacuated glass tube, resulting in the appearance of a cathode ray—a beam of electrons.

• Cathode Ray: The ray is composed of negatively charged particles (electrons).

• Properties:

  • Independent of the composition of the material from which they originate.

  • Carry a negative electrical charge.

  • Have low mass.

  • Can be deflected by electric and magnetic fields.

• Conclusion: The cathode ray is made of electrons. By using electric and magnetic fields, Thomson determined the charge-to-mass ratio of an electron.

Example: The cathode ray tube experiment is the basis for the development of television and computer screens.