Atomic Theory

Vocabulary flashcards covering the key terms and concepts from the lecture on atomic theory, from Democritus through Chadwick and Bohr.

Atom

The basic unit of matter; all matter is made of atoms.

Atomos

Greek word meaning 'not to be cut'; Democritus' term for the smallest piece of matter.

Democritus

Greek philosopher (460–370 B.C.) who proposed atoms exist and introduced the solid sphere idea.

Solid sphere model

Democritus' atomic model where atoms are indivisible, solid spheres.

Indivisible

Not able to be divided into smaller parts; a key assumption of Democritus' atoms.

Infinite atoms

Democritus believed atoms are infinite in number, always moving and able to join together.

Dalton's atomic theory

All matter is made of atoms; atoms of an element are identical; atoms of different elements differ; compounds form when atoms combine in fixed ratios; atoms rearranged in reactions.

Billiard-ball model

Dalton’s early model of atoms as solid, indivisible spheres.

Conservation of mass

In chemical reactions, atoms are neither created nor destroyed.

Law of constant composition

Elements combine in fixed ratios to form compounds.

Subatomic particles

Particles smaller than the atom, including electrons, protons, and neutrons.

Electron

Negatively charged subatomic particle; discovered by J.J. Thomson.

J.J. Thomson

Scientist who proposed the plum pudding model and discovered the electron.

Plum pudding model

Thomson’s model in which a positively charged 'pudding' contains embedded electrons.

Rutherford gold foil experiment

1911 experiment showing some alpha particles deflected by a dense center, indicating a small, positively charged nucleus.

Nucleus

Small, dense, positively charged center of the atom.

Nuclear model

Rutherford's model with a central nucleus and electrons surrounding it.

Planetary model

Another name for Rutherford's model, describing electrons orbiting the nucleus like planets.

Bohr model

1913 refinement where electrons occupy fixed energy orbits around the nucleus; energy depends on orbit size; radiation occurs when moving between orbits.

Energy levels / Orbits

Set sized orbits with specific energies; smaller orbits have lower energy.

Radiation

Absorption or emission of energy when an electron moves between orbits.

James Chadwick

Physicist who confirmed the existence of the neutron in 1932.

Neutron

Electrically neutral subatomic particle with mass similar to a proton.

Proton

Positively charged subatomic particle located in the nucleus; its discovery by Thomson was not achieved.

Atomic number

Number of protons in the nucleus; identifies the element.

Atomic mass

Mass of an atom; related to the sum of protons and neutrons.

Quantum physics

Advanced physics that led to abandoning the classical planetary model in favor of quantum models.

Rutherford-Bohr model

Hybrid reference to the combination of Rutherford’s nucleus concept with Bohr’s quantized orbits.