Atomic Theory Development

Overview

This lecture introduces the historical development of atomic theory, focusing on the contributions and ideas of Democritus and John Dalton.

Democritus (400 BCE) wondered what happens if matter is cut into smaller pieces repeatedly.
He proposed that matter is made of tiny, indivisible particles called "atomos" (meaning uncuttable).
Democritus believed atomos move through empty space and have different shapes based on the type of matter.
His idea was only philosophical and lacked experimental evidence.
Other philosophers like Aristotle rejected Democritus' idea, believing matter was continuous and made of four elements: earth, water, air, and fire.
Aristotle’s influence led most people to accept the four-element theory for nearly 2,000 years.

In the early 1800s, John Dalton used scientific experiments with gases to study matter.
Dalton revived the atomic idea, describing atoms as solid, indivisible spheres (solid sphere or billiard ball model).
Dalton provided scientific evidence, unlike Democritus.
Dalton’s postulates:
- All matter is made of tiny, indivisible atoms.
- Atoms of the same element are identical; atoms of different elements differ.
- Atoms combine in fixed ratios to form compounds.
- In chemical reactions, atoms are rearranged but do not change into other atoms or disappear.

Both believed matter is made of tiny, indivisible atoms.
Democritus imagined atoms had different shapes; Dalton described them as identical solid spheres for each element.
Democritus relied on philosophy; Dalton used experiments and scientific evidence.

Atomos — The smallest indivisible piece of matter, as proposed by Democritus.
Atom — A tiny, indivisible particle making up all matter, as described by Dalton.
Solid Sphere Model — Dalton’s idea of atoms being solid, uniform spheres.
Four-Element Theory — Aristotle’s belief that all matter is made of earth, water, air, and fire.