HISTORY OF ATOMS

Vocabulary flashcards covering key scientists, experiments, models, and fundamental terms related to the history of atomic theory and basic atomic structure.

Empedocles

Ancient Greek philosopher who proposed that all matter is composed of four elements: fire, air, water, and earth.

Four Classical Elements

Fire, air, water, and earth—the basic substances that Empedocles believed made up all matter.

Aristotle's Aether

The fifth, spiritual element added by Aristotle, thought to fill the heavens and enable substances to transform.

Democritus' Atomos

Idea that matter is made of tiny, indivisible particles called “atomos,” meaning indivisible.

John Dalton

Scientist who formulated the first modern atomic theory, viewing atoms as solid spheres unique to each element.

Dalton's Atomic Theory

States that atoms are indivisible, atoms of the same element are identical, atoms combine in whole-number ratios, and reactions rearrange atoms.

J. J. Thomson

Physicist who discovered the electron using cathode-ray tubes and proposed the plum pudding model.

Cathode Rays

Streams of tiny, negatively charged particles observed in gas discharge tubes; later identified as electrons.

Electron

Negatively charged subatomic particle discovered by J. J. Thomson; occupies regions outside the nucleus.

Plum Pudding Model

Thomson’s model of the atom depicting electrons embedded in a diffuse, positively charged sphere.

Ernest Rutherford

Physicist who performed the gold foil experiment and concluded that atoms contain a small, dense, positive nucleus.

Gold Foil Experiment

Rutherford’s experiment that showed most alpha particles passed through foil, leading to discovery of the nucleus.

Nucleus

Central, dense region of an atom containing protons and neutrons and holding most of the atom’s mass.

Neils Bohr

Scientist who proposed the planetary model with electrons in quantized energy levels around the nucleus.

Planetary Model

Bohr’s atomic model where electrons orbit the nucleus in fixed shells like planets around the Sun.

Quantized Energy Levels

Specific energy values an electron can possess; electrons jump between levels by absorbing or releasing energy.

James Chadwick

Physicist who discovered the neutron, an uncharged particle within the nucleus.

Neutron

Neutral subatomic particle in the nucleus that contributes to atomic mass but not charge.

Erwin Schrödinger

Physicist who developed the quantum mechanical model treating electrons as matter waves.

Quantum Mechanical Model

Modern atom model describing electron positions as probability clouds (orbitals) rather than fixed paths.

Subatomic Particles

Particles that compose atoms: protons (positive), neutrons (neutral), and electrons (negative).

Proton

Positively charged subatomic particle located in the nucleus; defines the atomic number of an element.

Element

Substance composed of only one type of atom and cannot be broken into simpler substances.

Ion

Atom or molecule that has gained or lost electrons, acquiring an overall electric charge.

Cation

Positively charged ion formed when an atom loses one or more electrons.

Anion

Negatively charged ion formed when an atom gains one or more electrons.

Isotope

Atoms of the same element with identical proton numbers but different neutron numbers, giving different masses.

Octet Configuration

Stable electron arrangement with eight electrons in the outermost shell, characteristic of noble gases.

Noble Gases

Group of elements with full outer electron shells, making them chemically inert and unreactive.

Plum Pudding Positive Charge

Diffuse positive matter in Thomson’s model balancing the embedded electrons.