Overview
This lecture reviewed the historical development of atomic theory, explained the structure of the atom, identified subatomic and elementary particles, and described their properties and significance.
History of Atomic Theory
- Democritus introduced the concept of "atomos," meaning indivisible particles.
- Aristotle proposed that matter is made of earth, water, air, fire, and ether.
- Dalton's atomic model described atoms as solid, indivisible spheres (billiard ball model).
- J.J. Thomson discovered electrons, proposing the "plum pudding" or "raisin bun" model.
- Rutherford's gold foil experiment showed atoms have a dense, positive nucleus (nuclear model).
- Bohr introduced energy levels for electrons, described as the "planetary model."
- Chadwick discovered neutrons, solving the repulsion mystery in the nucleus.
- Modern atomic theory includes the electron cloud model, using quantum principles.
Structure of the Atom
- Atom is the smallest particle of an element, not of matter overall.
- Atoms consist mostly of empty space, with a tiny, dense nucleus at the center.
- The nucleus contains protons (positive) and neutrons (neutral); electrons (negative) orbit the nucleus.
- Atoms are neutral by default, having equal numbers of protons and electrons.
- The nucleus is responsible for nearly all the atom’s mass; electron mass is negligible.
Subatomic and Elementary Particles
- Subatomic particles: protons, neutrons, and electrons.
- Protons and neutrons are nucleons (particles inside the nucleus).
- Protons have a positive charge, neutrons have zero charge, electrons have a negative charge.
- Protons and neutrons are made of even smaller particles called quarks.
- Quarks: Up quark (+2/3 charge), Down quark (−1/3 charge).
- Proton: made of two up quarks and one down quark (net charge +1).
- Neutron: made of one up quark and two down quarks (net charge 0).
- Electrons are elementary particles (not made of quarks).
Key Terms & Definitions
- Atom — Smallest particle of an element, composed of protons, neutrons, and electrons.
- Subatomic Particles — Particles smaller than atoms: protons, neutrons, electrons.
- Nucleus — Dense center of an atom containing protons and neutrons.
- Nucleon — A particle found in the nucleus (proton or neutron).
- Quark — Fundamental constituent of protons and neutrons; types include up and down.
- Electron Cloud Model — Atomic model describing electrons as a cloud around the nucleus.
- Elementary Particle — A particle not made of smaller components (e.g., quarks, electrons).
Action Items / Next Steps
- Review the video lesson on the history of modern atomic theory.
- Prepare for the next session by revising subatomic and elementary particles.