Atoms Through Time – Key Vocabulary

Vocabulary flashcards summarizing the scientists, particles, and theories that shaped the atomic model from ancient Greece to the 20th century.

Democritus

Ancient Greek philosopher (c. 400 BC) who first proposed that all matter is made of indivisible, indestructible particles called “atomos.”

Atomos

Greek term meaning “uncuttable”; Democritus’s name for the smallest, indestructible units of matter—atoms.

John Dalton

English chemist (early 1800s) who formulated the first modern atomic theory, asserting that matter is composed of identical, indivisible atoms unique to each element.

Dalton’s Atomic Theory

States that all matter is made of indivisible, indestructible atoms; atoms of the same element are identical in mass and properties.

Dmitri Mendeleev

Russian chemist who organized elements by atomic mass into the first Periodic Table, predicting undiscovered elements’ properties.

Periodic Table of Elements

Tabular arrangement of chemical elements devised by Mendeleev, grouping elements with similar properties and predicting new ones.

J.J. Thomson

British physicist who discovered the electron in 1897 through cathode-ray experiments, earning a Nobel Prize and modernizing atomic physics.

Electron

Negatively charged subatomic particle discovered by J.J. Thomson; fundamental to chemical bonding and electricity.

Max Planck

German physicist who introduced quantum theory, showing that energy is emitted or absorbed in discrete amounts (quanta); Nobel Prize 1918.

Quantum Theory

Planck’s concept that energy changes occur in fixed quanta rather than continuous amounts, laying groundwork for quantum mechanics.

Ernest Rutherford

New Zealand–born British physicist who discovered the proton (1919), proposed the nuclear model of the atom, and won the 1908 Nobel Prize in Chemistry.

Proton

Positively charged subatomic particle located in the atomic nucleus; discovered by Ernest Rutherford.

James Chadwick

English physicist who discovered the neutron in 1932, earning the 1935 Nobel Prize in Physics.

Neutron

Electrically neutral subatomic particle residing in the nucleus alongside protons; discovered by James Chadwick.