Lecture Notes: The Atom
Historical Background
- 5th Century BC (Ancient Greece):
- Democritus: Early philosopher who theorized about the concept of the atom.
- Idea not widely accepted by contemporaries.
- Late 1700s (England):
- John Dalton:
- Revived and formalized the concept of the atom.
- Known as the father of modern chemistry.
- Developed the atomic theory through four postulates.
Dalton's Atomic Theory
- First Postulate: Each element is made up of tiny, indivisible particles called atoms.
- Originally thought atoms were indivisible.
- Modern understanding: Atoms consist of neutrons, protons, and electrons.
- Second Postulate: Atoms of a given element are identical; different elements have different kinds of atoms.
- Third Postulate: Chemical compounds are formed when atoms of different elements combine in fixed ratios.
- Examples: Carbon + Oxygen = Carbon Dioxide or Carbon Monoxide.
- Fourth Postulate: Chemical reactions involve reorganization of atoms, not changing the atoms themselves.
Discovery of Electrons (Late 1800s)
- J.J. Thompson:
- Conducted experiments with cathode ray tubes.
- Observed streams of particles moving from negative (cathode) to positive (anode) electrodes.
- Applied an external electric field to deflect cathode rays.
- Discovered that cathode rays consisted of negatively charged particles, later named electrons.
Key Findings from Thompson's Experiments
- Electrons are negatively charged and can be emitted from metals.
- Determined charge-to-mass ratio of electrons: 1.76 x 10^8 coulombs/gram.
- Experiments were reproducible across different metals, indicating that electrons are fundamental to all atoms.
- Led to the concept of electrical neutrality in atoms, suggesting the existence of positive charges.
The Plum Pudding Model
- Proposed by J.J. Thompson:
- Atoms consist of a diffuse cloud of positive charge.
- Electrons are embedded within this positive "cloud."
- This model helped in understanding the neutral charge of atoms.
Note: While Dalton's early views on indivisibility were later disproven, his foundational work paved the way for the development of modern atomic theory and subsequent discoveries such as electrons.