Atomic Structure and Historical Atomic Models

Practice vocabulary flashcards covering subatomic particles, isotope principles, early atomic theorists, and fundamental experiments in atomic physics and chemistry.

Atom

The smallest unit of an element that can exist either alone or in combination with other such particles of the same or different elements.

Proton

A dense subatomic particle located in the nucleus with a positive charge ($+1$) and a mass approximately equal to $1\,\text{u}$ (atomic mass unit).

Neutron

A subatomic particle located in the nucleus that carries no electrical charge (neutral) and has a mass approximately equal to that of a proton ($1\,\text{u}$).

Electron

A negatively charged ($-1$) subatomic particle located outside the nucleus with a very small mass of approximately $\frac{1}{2000}$ of the mass of a proton or neutron.

Nucleus

The extremely dense, small, positively charged region at the center of an atom that contains nearly all of the atom's mass, consisting of protons and neutrons.

Atomic Number

The total number of protons found in the nucleus of an atom, which uniquely defines the identity of an element.

Mass Number

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

Isotopes

Atoms of the same element that have the same number of protons (atomic number) but different numbers of neutrons, resulting in different mass numbers.

Average Atomic Mass

The weighted average of the atomic masses of the naturally occurring isotopes of an element, calculated using each isotope's mass and percent abundance.

Orbital

A three-dimensional region of space around the nucleus that represents the most probable location of finding an electron.

Ion

An atom or group of bonded atoms that has acquired an electrical charge by either losing or gaining one or more electrons.

Democritus

The ancient Greek philosopher who first formulated the idea of tiny, indivisible fundamental particles of matter, which he termed 'atomos'.

John Dalton

The English chemist who proposed the first scientific atomic theory to explain the fundamental laws of chemical combination.

Dalton's Atomic Theory

A foundational scientific theory stating that all matter consists of indivisible atoms, atoms of a given element are identical in mass and properties, atoms cannot be created or destroyed, and chemical reactions involve combinations, separations, or rearrangements of atoms.

Cathode Rays

Streams of fast-moving, negatively charged electrons emitted from the negative cathode toward the positive anode in an evacuated tube and deflected away from a negative electrical plate.

J.J. Thomson

The scientist who measured the charge-to-mass ratio of cathode-ray particles to discover the electron and proposed the plum pudding model of the atom.

Plum Pudding Model

An early atomic model proposed by J.J. Thomson in which negative electrons were thought to be embedded within a uniform sphere of positive charge.

Eugene Goldstein

The scientist credited with the discovery of positively charged particles (protons) in cathode-ray tube experiments.

James Chadwick

The English physicist who experimentally confirmed the existence of the neutron in the atomic nucleus.

Ernest Rutherford

The physicist who directed the gold foil experiment, discovering the dense, positively charged atomic nucleus and demonstrating that the atom is mostly empty space.

Gold Foil Experiment

An experiment in which alpha particles were directed at thin gold foil; because the majority passed straight through while a small fraction deflected at large angles, scientists concluded that the atom consists largely of empty space surrounding a tiny, dense, positively charged nucleus.

Alpha Particle

A positively charged particle consisting of two protons and two neutrons ($\phantom{}^{4}_{2}\text{He}$) that was fired at metal foil in Rutherford's scattering experiments.

Postulate

A fundamental principle or claim suggested or proposed without proof to serve as the basis for a theory or chain of reasoning.

Law of Conservation of Mass

A law of chemistry stating that mass is neither created nor destroyed in chemical reactions, meaning the total mass of the reactants equals the total mass of the products.