Beginnings and Evolution of the Atomic Theory of Matter

Comprehensive vocabulary flashcards covering the historical development and modern concepts of atomic theory, spanning classical atomism, Dalton's postulates, historical atomic models, spectroscopy, and quantum mechanical subshells.

Atomos

The Greek word meaning "uncuttable," proposed by Leucippus of Miletus and Democritus of Abdera around 500 BCE to describe tiny, indivisible, perfectly solid particles of matter moving continuously within a void.

Aristotle's Elemental Theory

The ancient philosophical doctrine contending that everything in the world is composed solely of four fundamental elements: earth, air, fire, and water.

Dalton's Atomic Theory

A theory proposed in 1803 by English chemist John Dalton asserting that matter consists of indivisible spherical atoms, that atoms of a given element have identical properties and mass, and that atoms combine in simple whole-number ratios to form compounds.

Law of Conservation of Mass

A fundamental law proposed by Antoine Lavoisier in 1789 stating that the total mass of a closed system remains constant during physical transformations or chemical reactions, meaning matter is neither created nor destroyed.

Law of Constant Composition

Also known as the law of definite proportions, formulated by Joseph-Louis Proust in 1794, stating that a chemical compound always contains the same mass ratio of its constituent elements regardless of its source.

Electrons

Negatively charged subatomic particles discovered inside the atom by British physicist J.J. Thomson in 1897 using cathode ray tube experiments (a term previously coined by George Johnstone Stoney in 1891).

Plum Pudding Model

An atomic model proposed in 1904 by J.J. Thomson in which negatively charged electrons (the plums) are uniformly embedded in a positively charged, jellylike spherical blob (the pudding) to yield an electrically neutral atom.

Gold Foil Experiment

A 1909 experiment conducted by Ernest Rutherford, Hans Geiger, and Ernest Marsden where thin gold sheets were bombarded with positively charged alpha particles, demonstrating that atoms consist mostly of empty space with a dense, positively charged center.

Nuclear Model

An atomic model proposed in 1911 by Ernest Rutherford establishing that an atom consists mostly of empty space surrounding a tiny, dense, positively charged nucleus with a radius approximately $\frac{1}{100\,000}$ that of the atom itself.

Proton

A positively charged subatomic particle identified by Ernest Rutherford inside the dense atomic nucleus.

Neutron

An uncharged subatomic particle discovered in the atomic nucleus by British physicist James Chadwick in 1932.

Line Spectrum

A diagram composed of discrete vertical lines of distinct colors against a dark background, where each colored line corresponds to a specific wavelength and energy of electromagnetic radiation emitted by an atom.

Planetary Model

An atomic model developed in 1913 by Niels Bohr proposing that electrons travel in distinct, concentric circular orbits of quantized energy around the nucleus, analogous to planets orbiting the sun.

Ground State

The lowest possible energy state of an atom in which electrons reside in their closest, default orbits around the nucleus.

Excited State

A temporary, unstable condition achieved when an atom absorbs energy, causing an electron to jump to a higher-energy orbit farther from the nucleus.

Quantum Mechanical Model

The modern atomic model that incorporates the wavelike behavior of electrons and mathematical probabilities to describe electrons residing within three-dimensional electron clouds rather than fixed planar orbits.

Atomic Orbitals

Specific regions within an electron shell where there is the highest probability of locating an electron, derived from Erwin Schrödinger's 1926 wave equation.

Principal Shell Number ($n$)

A positive nonzero integer ($n = 1, 2, 3, \dots$) representing an electron shell; larger values correspond to electron shells with higher energy that are situated farther from the nucleus.

Subshell

A division of an electron shell designated by an integer $l$ ranging from $0$ to $n - 1$, grouping atomic orbitals of identical type ($s$, $p$, $d$, or $f$).

$s$ Subshell

The subshell corresponding to $l = 0$, containing exactly one $s$ atomic orbital.

$p$ Subshell

The subshell corresponding to $l = 1$, consisting of three distinct $p$ atomic orbitals.

$d$ Subshell

The subshell corresponding to $l = 2$, containing five distinct $d$ atomic orbitals.

$f$ Subshell

The subshell corresponding to $l = 3$, containing seven distinct $f$ atomic orbitals.

Quantum Dots

Semiconducting nanocrystals whose electronic, optical, and magnetic properties vary according to their physical dimensions, recognized by the 2023 Nobel Prize in Chemistry awarded to Moungi Bawendi, Louis Brus, and Alexei Ekimov.

Electron Ptychography

An advanced imaging technique using electron bombardment that allowed Cornell University researchers in 2021 to produce ultra-high-resolution images of atoms within a praseodymium orthoscandate crystal.