Atomic History

Democritus (460–370 BC)

First person to propose the idea of atoms; believed matter was made of indivisible particles called "atomos".

Aristotle (384–322 BC)

Opposed atomism; argued matter was made of 4 elements (Fire, Air, Earth, Water). His debate skills delayed atomic theory for over 2,000 years.

4 Elements Theory

Aristotle's model stating substances are blends of Fire (hot), Air (light), Earth (cool/heavy), and Water (wet).

Antoine Lavoisier (1743–1794)

Father of Modern Chemistry; discovered the Law of Conservation of Mass in 1789.

Law of Conservation of Mass

Principle that mass is neither created nor destroyed during standard physical or chemical changes.

Joseph Proust (1754–1826)

Discovered the Law of Definite Proportions in 1799.

Law of Definite Proportions

Principle that a chemical compound always contains the exact same proportion of elements by mass regardless of sample size or source.

Law of Multiple Proportions

Principle that when elements combine to form more than one compound, the mass ratios of the second element compare in small whole numbers.

John Dalton (1766–1844)

"Father of Atomic Theory"; English schoolteacher who proposed the first scientific atomic theory in 1803 to explain mass laws.

Dalton's Atomic Model

"Tiny Ball" or "Solid Sphere" model: atoms are tiny, solid, indivisible spheres with no internal structure.

5 Postulates of Dalton's Theory

1. Matter made of indivisible atoms. 2. Identical atoms per element. 3. Atoms indestructible/uncreatable. 4. Combine in simple ratios. 5. Rearranged in chemical reactions.

Flaws in Dalton's Theory

Atoms ARE divisible into subatomic particles (protons, neutrons, electrons), and atoms of the same element CAN differ in mass (isotopes).

J.J. Thomson (1856–1940)

Discovered the electron using cathode ray tubes, proving the atom is divisible.

Thomson's Experiments

Proved cathode rays had mass (turned paddle wheel), negative charge (deflected to + plate), and existed in all metal electrodes.

Thomson's "Plum Pudding" Model

Model featuring negative electrons scattered throughout a uniform sphere of positive charge.

Robert Millikan (1909)

Conducted the Oil Drop Experiment; measured the electron's charge (

Ernest Rutherford (1871–1937)

Conducted the Gold Foil Experiment; discovered the dense, positively charged nucleus and proved the atom is mostly empty space.

Gold Foil Experiment Findings

Most alpha particles passed straight through the foil, but 1 in 8,000 bounced back at ~180° after hitting the dense nucleus.

Rutherford

Bohr Planetary Model

Flaw in Rutherford's Model

Classical physics predicted orbiting electrons should lose energy and crash into the positive nucleus, destroying the atom.

Eugen Goldstein (1886)

Discovered positively charged particles (protons) in cathode ray tubes moving opposite to cathode rays.

James Chadwick (1932)

Discovered the neutral subatomic particle (neutron), which is heavy enough to split a nucleus and enabled the atomic bomb.

Subatomic Particle Properties

Electron (e

Atomic Number vs. Mass Number

Atomic Number (Z) = number of protons (identifies element); Mass Number (A) = protons + neutrons.

Isotopes & Average Atomic Mass

Isotopes are atoms of the same element with different neutron numbers; Average Atomic Mass is the weighted average of all natural isotopes.