Historical Development of Atomic Theory

Introduction to Subatomic Particles

• Subatomic particles are smaller than atoms.

• Historically, atoms were believed to be the smallest unit of matter.

Historical Background

• The understanding that atoms might not be the smallest unit emerged in the late 1800s.

• Physicists began exploring electricity and its relation to atomic structure.

• Initial thought: Charged atoms were responsible for electric current; however, this was proven incorrect.

J.J. Thomson's Experiments

• In 1897, J.J. Thomson conducted experiments using a cathode ray tube:

  • A cathode ray tube consists of an evacuated glass tube with two metal electrodes on one side and another pair in the middle.

  • When voltage is applied, the glass opposite the negative electrode glows.

Observations from the Cathode Ray Tube

• Thomson noted that cathode ray particles bent towards a positive charge and away from a negative charge.

• Conclusions derived from observations:

  • Particles are attracted to positive charges, indicating their negative charge.

  • Because atoms are overall neutral, the existence of negatively charged particles implied that atoms contain these subatomic particles.

Discovery of Electrons

• Thomson identified the particles as electrons, concluding:

  • Electrons are negatively charged.

  • They exhibited a charge to mass ratio smaller than that of any known atom, signaling they are components of atoms.

Impact on Atomic Theory

• Thomson's findings revolutionized the understanding of atomic structure.

• Resulted in new theories:

  • Plum Pudding Model: Suggests electrons are embedded in a positively charged sphere.

  • Later evolved into the Electron Cloud Model: Portrays electrons in probabilistic distributions around the nucleus.

Conclusion

• The discovery of subatomic particles fundamentally changed the landscape of chemistry and physics, leading to a deeper understanding of matter's structure.