Atomic Theory Evolution

Overview

This lecture traces the history and evolution of atomic theory, from ancient Greece to the development of the modern quantum model.

Around 440 BCE, Democritus proposed that all matter is made of indivisible particles called "atomos."
Democritus suggested these particles differ in size and shape for each substance.
Aristotle disagreed, claiming matter was made of four elements: earth, wind, water, and fire.
Democritus’s atomic idea was largely ignored in favor of Aristotle's for centuries.

In 1808, John Dalton revived atomic theory with experimental evidence.
Dalton showed that substances break down into the same elements in fixed proportions.
He concluded compounds are combinations of different atoms, each with specific size and mass, which cannot be created or destroyed.

In 1897, J.J. Thomson discovered the electron, a negatively charged subatomic particle.
Thomson’s "chocolate chip cookie" model had electrons embedded in a sphere of positive matter.
Thomson won the Nobel Prize in 1906 for his discovery.

Ernest Rutherford’s gold foil experiment showed most of the atom is empty space.
He discovered atoms have a dense, positively charged nucleus at their center.
Positively charged alpha particles occasionally bounced back after striking the nucleus.

In 1913, Niels Bohr proposed electrons orbit the nucleus at fixed energies and distances.
Electrons can jump between energy levels but not exist in between them.

Electrons behave both as particles and as waves, not confined to a single point.
Werner Heisenberg's uncertainty principle states we cannot know both position and speed of an electron.
The quantum model describes electrons existing within a range of possible locations.

Electrons shifting energy levels release or absorb energy as specific light wavelengths, producing colors like those in fireworks.

Atomos — Greek for indivisible; original term for atoms by Democritus.
Electron — Negatively charged subatomic particle discovered by J.J. Thomson.
Nucleus — Dense, positively charged center of the atom discovered by Rutherford.
Energy Level — Fixed orbit where electrons reside around the nucleus as per Bohr.
Uncertainty Principle — Heisenberg’s idea that position and speed of electrons can’t be determined simultaneously.
Quantum Model — Current model describing electrons as existing in probabilistic locations.

Review the different atomic models and their key features.
Study the historical timeline of atomic theory advancements.
Prepare examples of how energy level changes produce light in everyday phenomena.