History of Atomic Theory

Vocabulary flashcards covering the key historical figures, experiments, models, and concepts in the development of atomic theory from ancient Greece to the modern quantum model.

Atomos

A term meaning uncuttable or indivisible, coined 2500 years ago by Greek philosopher Leucippus and his pupil Democritus to describe the ultimate particles of matter.

Antoine Lavoisier

The French chemist who proposed the law of conservation of mass, stating that matter's mass remains the same even if it changes shape or form.

James Dalton

The English teacher who determined that chemical elements exist as discreet packets of matter.

Discharge tubes

Gas-filled tubes containing electrodes at each end that emit light when an electrical current passes through them.

Cathode ray

A negatively charged ray of light produced by a negative electrode (cathode) inside a discharge tube.

Eugen Goldstein

A German physicist who discovered in 1886 that discharge tubes emit rays from the positive electrode in the opposite direction, indicating matter contains a positive charge.

Corpuscles

The name given by J.J. Thompson to the small, negatively charged particles—about 1000 times lighter than hydrogen—that are now called electrons.

Plum pudding model

J.J. Thompson's atomic model that envisioned negatively charged electrons randomly distributed within a positively charged matrix, similar to fruit in an English cake.

Alpha particles

Very small, positively charged particles produced by the decay of radium, utilized by Ernest Rutherford in his gold foil experiment.

Nucleus

The concentrated, very small central area of an atom identified by Ernest Rutherford that holds the atom's entire positive charge.

Proton

A fundamental, positively charged particle discovered by Ernest Rutherford after bombarding nitrogen with alpha particles and producing hydrogen ions.

Niels Bohr

A physicist who combined Rutherford's experimental results with the mathematical models of Max Planck and Albert Einstein to predict electron positions around the nucleus.

Planetary model

Niels Bohr's atomic model portraying electrons in specific circular orbits around a small central nucleus, with each orbit corresponding to a particular energy level.

Werner Heisenberg

A German theoretical physicist who discovered that it is impossible to determine with certainty both the momentum and the exact position of an electron or subatomic particle.

Orbitals

Regions around the atomic nucleus (designated by names such as s, p, d, and f) where an electron is mathematically most likely to be found.

Cloud model

The modern quantum model of the atom, which depicts electron arrangements as probability clouds where the intensity of color represents the likelihood of finding an electron.