Atomic Models and Atomic Theory

Comprehensive vocabulary flashcards summarizing key terms, models, quantum numbers, subatomic particles, and laws from the history and modern understanding of atomic theory.

Atomos

Tiny, invisible particles moving through a void or vacuum that Leucippus and Democritus proposed as the fundamental constituents of matter.

De rerum natura

A poem written in $50\,\text{BCE}$ by the Latin poet and philosopher Lucretius (translated as "On the nature of things") that praised atomism and preserved the ideas of Leucippus and Democritus.

Solid Sphere Model

The atomic model proposed by John Dalton in $1803$, which defines atoms as dense, solid, indestructible spheres with mass and no internal subatomic structure.

Law of Conservation of Mass

A fundamental law of chemistry stating that during a chemical reaction, there is no change in the total mass of the substances involved before and after the reaction.

Law of Definite Composition

A chemical law stating that a given chemical compound always contains the exact same elements in the exact same percentage by mass.

Law of Multiple Proportions

A chemical law stating that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in a ratio of small whole numbers (such as $1:1$, $1:2$, or $2:3$).

Plum Pudding Model

The atomic model proposed by J.J. Thomson in $1897$, describing the atom as a uniform positively charged sphere in which negatively charged electrons are loosely embedded.

Cathode Ray Tube Experiment

An experiment performed by J.J. Thomson where electrical currents passed through gas at low pressure produced a cathode ray deflected by electric and magnetic fields, proving the existence of electrons.

Nuclear Model

The atomic model introduced by Ernest Rutherford in $1911$, proposing that an atom consists of a small, dense, positively charged nucleus at its center surrounded by orbiting electrons.

Gold Foil Experiment

An experiment by Ernest Rutherford where alpha particles were fired at thin gold foil; most passed through undeflected, but a few deflected at large angles, showing the atom is mostly empty space with a dense nucleus.

Atomic Number ($Z$)

The number of protons found within the nucleus of an atom, which uniquely determines the element's identity.

Mass Number ($A$)

The total number of protons ($Z$) and neutrons ($n$) combined inside an atom's nucleus.

Neutron

An electrically neutral subatomic particle in the nucleus discovered by James Chadwick in $1932$, having a mass approximately equal to that of a proton.

Subatomic Particles

The primary structural units of an atom, comprising positively charged protons ($p^+$), neutral neutrons ($n^0$), and negatively charged electrons ($e^-$).

Planetary Model

The atomic model proposed by Niels Bohr in $1913$, where electrons travel in quantized, circular energy levels around the nucleus and emit or absorb energy only when moving between states.

Stationary State

A defined orbit in Bohr's model where an electron maintains a fixed amount of energy without absorbing or radiating energy.

Ground State

The lowest possible energy state in which an electron naturally resides.

Excited State

A state of higher energy achieved when an electron absorbs energy from an external source and jumps to a higher level.

Quantum Mechanical Model

The modern atomic model proposed by Erwin Schrödinger in the $1920\text{s}$, treating electrons as wave-like probability distributions rather than particles in fixed orbits.

Atomic Orbital

A three-dimensional region in an atom where there is a high probability of finding an electron, visualized as a electron cloud.

Electron Cloud Model

The visual representation of an electron's location in quantum mechanics as a cloud whose density corresponds to electron probability.

Principal Quantum Number ($n$)

The quantum number that describes the main energy level and relative size of an atomic orbital ($n = 1, 2, 3...$).

Azimuthal Quantum Number ($l$)

Also called the orbital angular momentum quantum number, it defines the shape of a subshell or orbital, taking integer values from $0$ to $n - 1$ ($s, p, d, f$).

Magnetic Quantum Number ($m_l$)

The quantum number specifying the orientation of an orbital in space relative to other orbitals, ranging in value from $-l$ to $+l$.

Spin Quantum Number ($m_s$)

The quantum number describing the direction of an electron's intrinsic spin axis, taking values of $+\frac{1}{2}$ (spin-up) or $-\frac{1}{2}$ (spin-down).

Electron Configuration

The notation showing how electrons are distributed among the various sublevels and energy levels of an atom.

Aufbau Principle

The principle stating that electrons occupy available atomic orbitals one by one, filling the lowest energy orbital first before moving to higher energy orbitals.

Valence Electrons

The electrons located in the outermost sublevel or shell of an atom.

Hund's Rule

The rule dictating that when electrons enter orbitals of equal energy within a sublevel, they fill empty orbitals individually with parallel spins before pairing up.