John Dalton’s Atomic Theory & The Development of Atomic Structure

Vocabulary flashcards covering foundational laws, historic scientists, atomic models, and key subatomic concepts from the lecture notes on Dalton’s Atomic Theory and the evolution of atomic structure.

Atom

The basic, smallest unit of matter; originally proposed as an indivisible particle by ancient Greek philosophers.

Atomos

Greek word meaning “indivisible”; the root of the word atom, coined by Democritus.

Atomist

Supporter of Leucippus and Democritus’ idea that all matter is composed of indivisible atoms.

Law of Conservation of Mass

States that matter (and thus mass) can neither be created nor destroyed in a chemical reaction.

Law of Constant Composition (Law of Definite Proportions)

Different samples of a given compound always contain the same elements in the same mass ratio.

Law of Multiple Proportions

When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in small-whole-number ratios.

Dalton’s Atomic Theory

Early 19th-century theory stating that (1) all matter is made of atoms, (2) atoms of a given element are identical and different from those of other elements, (3) compounds form from fixed ratios of atoms, and (4) chemical reactions involve rearrangement of atoms without their creation or destruction.

Billiard Ball Model

John Dalton’s visualization of the atom as a solid, indivisible, spherical particle.

Plum Pudding Model

J.J. Thomson’s atomic model in which electrons (negatively charged “plums”) are embedded in a positively charged “pudding.”

Cathode Ray Experiment

Thomson’s investigation showing all atoms contain tiny, negatively charged particles (electrons).

Nuclear Model

Ernest Rutherford’s post-gold-foil visualization of the atom as mostly empty space with a dense, positively charged nucleus.

Gold Foil Experiment

Rutherford’s experiment that fired alpha particles at gold foil, leading to discovery of the atomic nucleus.

Planetary Model

Niels Bohr’s 1913 refinement placing electrons in fixed energy orbits around the nucleus, similar to planets around the Sun.

Quantum Mechanical Model

Modern atomic theory that treats electron positions as probability clouds (orbitals) rather than fixed paths; based on wave mechanics.

Electron Cloud Model

Schrödinger’s depiction of electrons as wave-like clouds of probability surrounding the nucleus.

Proton

Positively charged subatomic particle located in the nucleus; mass ≈ 1 atomic mass unit (a.m.u.).

Neutron

Electrically neutral subatomic particle in the nucleus; discovered by James Chadwick; mass ≈ 1 a.m.u.

Electron

Negatively charged subatomic particle located outside the nucleus in energy levels or orbitals.

Nucleus

Dense central core of the atom containing protons and neutrons; accounts for 99% of atomic mass yet is ~10,000 times smaller than the atom’s overall size.

Atomic Number (Z)

Number of protons (and, in a neutral atom, electrons); uniquely identifies an element.

Mass Number (A)

Total number of protons plus neutrons in an atom’s nucleus.

Periodicity

Regular recurrence of elemental properties at certain intervals on the periodic table.

Law of Octaves

John Newlands’ observation that every eighth element exhibits similar properties when elements are ordered by increasing atomic mass.

Henry Moseley

Physicist who correlated atomic number with X-ray frequencies (1913) and defined atomic number as the proton count.

John Dalton

English teacher-chemist (1766-1844) who pioneered modern atomic theory and proposed the billiard ball model.

Leucippus

Early Greek philosopher who first proposed the concept of indivisible particles called atoms.

Democritus

Student of Leucippus; coined the term atomos and argued that matter cannot be divided indefinitely.

J.J. Thomson

English physicist who discovered the electron (1897) and proposed the plum pudding model; Nobel laureate 1906.

Ernest Rutherford

Physicist whose gold foil experiment led to the nuclear model of the atom (1911).

Niels Bohr

Physicist who introduced quantized electron orbits in his planetary model (1913).

James Chadwick

Discovered the neutron (1932) by bombarding beryllium with alpha particles.

Dmitri Mendeleev

Russian chemist who arranged elements by recurring properties, leading to the first widely accepted periodic table.

Lothar Meyer

German chemist who independently developed a periodic classification of the elements similar to Mendeleev’s.