History and Development of Atomic Theory

Vocabulary practice flashcards covering the historical development of atomic models, major experiments, quantum mechanics, subatomic particles, and isotope properties.

Atomos

The Greek word meaning "indivisible" or "not to be cut," coined around 460 BC by Democritus to describe the tiny, uncuttable particles resulting from pounding materials with a mortar and pestle.

Aristotle's Four Elements Theory

The ancient philosophical theory accepted for nearly 2000 years asserting that all matter is composed of varying combinations of four fundamental elements: fire, water, earth, and air.

Solid Sphere Model (Billiard Ball Model)

John Dalton's 1808 atomic model proposing that matter is composed of tiny, indivisible, indestructible spheres capable of bouncing with perfect elasticity.

Cathode Rays

Streams of negatively charged particles (electrons) produced when an electric current passes through a gas inside an evacuated glass tube.

Plum Pudding Model

J.J. Thomson's 1898 model depicting negatively charged electrons (corpuscles) scattered unevenly throughout an elastic sphere of diffuse positive charge.

Gold Foil Experiment

Ernest Rutherford's 1910 experiment in which alpha particles ($\text{He}^{2+}$) were fired at a thin sheet of gold foil, revealing that while most passed straight through, a small fraction were deflected or bounced back.

Empty Space Model (Beehive Model)

Ernest Rutherford's atomic model stating that an atom consists largely of empty space with a dense central nucleus holding all positive charge and holding electrons in place by electrical attraction.

Planetary Model (Bohr Model)

Niels Bohr's 1913 atomic model proposing that electrons travel in distinct circular orbits at fixed energy levels around a central nucleus, analogous to planets orbiting the sun.

Ground State

The lowest possible energy level that an electron or atom can occupy.

Excited State

An unstable, higher energy state entered when an atom or electron absorbs a quantized amount of energy.

Millikan's Oil Droplet Experiment

The 1908 experiment conducted by Robert Millikan that measured the effect of an electric field on charged oil droplets to confirm that electrons carry a negative electrical charge.

Neutron

A neutral subatomic particle situated in the nucleus, discovered in 1932 by James Chadwick by analyzing alpha particles colliding with atomic nuclei.

Photoelectric Effect

The emission of electrons from a material when it absorbs energy from shortwave electromagnetic radiation, discovered by Heinrich Hertz.

Quantum of Energy

A single discrete packet or unit of energy that matter can absorb or emit, postulated by Max Planck to follow the relationship $E = hf$ (or $E = h\nu$).

Photon

A quantum packet of light energy introduced by Albert Einstein, who viewed electromagnetic radiation as a stream of localized particles.

Planck's Constant ($h$)

A fundamental physical constant equal to $6.63 \times 10^{-34}\,\text{J}\cdot\text{s}$, relating the energy of a quantum to the frequency of its radiation.

Quantum Mechanical Model

The modern mathematical model formulated by Schrödinger, de Broglie, and Heisenberg that describes electrons not by fixed paths, but as probability clouds (orbitals) of allowed energies around the nucleus.

Atomic Number ($Z$)

The total number of protons contained in the nucleus of an atom, defining the identity and chemical properties of an element.

Mass Number ($A$)

The combined total number of protons and neutrons situated in the nucleus of an atom.

Isotopes

Atoms of the same element containing the same number of protons but varying numbers of neutrons, giving them distinct mass numbers.

Radioisotope

An unstable isotope whose nucleus undergoes spontaneous radioactive decay, emitting subatomic particles and/or gamma rays.

Alpha Particle ($\alpha$)

A positively charged nuclear radiation particle equivalent to a Helium nucleus; it has the lowest penetrating capability among alpha, beta, and gamma radiation.

Gamma Ray ($\gamma$)

A high-energy form of electromagnetic radiation emitted during radioactive decay; it is the most penetrating among alpha, beta, and gamma emissions.