Atomic Structure: History of Atomic Models

Flashcards covering the historical development of atomic models, including Dalton's theory, Thomson's cathode ray experiments, Rutherford's gold foil experiment, Bohr's quantized energy levels, and the Wave-Mechanic model.

Atom

Derived from "a tom," meaning not breakable or indivisible.

Law of Conservation of Mass

A law formulated by Antoine Lavoisier in the 1770's stating that the mass of reactants equals the mass of products, demonstrating that pieces of matter are rearranged in a chemical reaction.

Law of Definite Proportions

A law established by Joseph Proust (1754–1826) in the 1790's stating that all samples of a compound are composed of the same elements in the same proportion by mass.

Law of Multiple Proportions

A law formulated by John Dalton in the 1800's stating that when two elements form different compounds, the ratio of one element in both compounds will be a small whole number ratio when the amount of the other element is fixed (e.g., $CO$ vs. $CO_2$ with oxygen in a $1:2$ ratio).

Dalton's Model of the Atom

An early atomic model describing the atom as a tiny, indivisible, solid sphere.

Dalton's Atomic Theory

A four-part theory stating: 1) elements are composed of indivisible particles called atoms; 2) all atoms of the same element are identical, particularly in mass; 3) atoms of different elements are different, particularly in their masses; and 4) compounds are atoms of different elements joined in small whole number ratios.

Electron

A negatively charged subatomic particle discovered by J J Thomson using a cathode ray tube with magnetic and electric fields, which is about $2000$ times smaller than a hydrogen atom.

Plum Pudding Model

An atomic model proposed by J J Thomson depicting the atom as a neutral sphere containing negative particles immersed in a positive medium.

Millikan Oil Drop Experiment

An experiment that successfully determined and discovered the charge on an electron.

Alpha Particle

A positive subatomic particle fired in Rutherford's experiments that has about the mass of helium but is much smaller.

Gold Foil Experiment

An experiment by Ernest Rutherford where alpha particles were fired at a thin sheet of gold foil; because most passed through while a few deflected, it proved the atom is mostly empty space with a small, relatively massive, positive nucleus orbited by electrons.

Rutherford's Planetary Model

An atomic model proposing that an atom is mostly empty space with a small, dense, positive nucleus, where electrons orbit the nucleus like planets around the sun.

Collapse Problem of Rutherford's Model

The theoretical issue where orbiting electrons should continuously slow down and collapse into the nucleus, which does not happen.

Bohr Model of the Atom

An atomic model proposed by Neils Bohr stating that electrons orbit the nucleus in defined energy levels, neither gaining nor losing energy as long as an electron remains within an energy level.

Ground State

The state in which electrons occupy the lowest available energy levels to remain as close to the nucleus as possible.

Excited State

A state that occurs when an electron gains enough energy to move to a higher energy level further from the nucleus, acquiring greater positional energy.

Photon Emission

The release of energy as electromagnetic radiation ($E = hf$) that occurs when an excited electron loses positional energy and returns to a lower energy level.

Hydrogen Emission Spectrum

The distinct set of spectral lines emitted by hydrogen in the visible region at wavelengths of $410\,\text{nm}$, $434\,\text{nm}$, $486\,\text{nm}$, and $656\,\text{nm}$.

Matter Waves

The wave nature of particles described by De Broglie using the relation $m = \frac{h}{c\lambda}$.

Wave-Mechanic Model

Also known as the Cloud Charge Model, Quantum Mechanic Model, or Orbital Model; it states that the atom is mostly empty space with a small, massive, positive nucleus containing protons and neutrons, while electrons move in standing waves across energy levels, sublevels, and orbitals.

Principal Energy Levels

Regions indicated by integers from $1$ to $7$ in ground-state atoms, where the maximum capacity of electrons on any level $n$ is given by $2n^2$.

Sublevels

Energy subdivisions indicated by the letters $s$, $p$, $d$, and $f$ in order of increasing energy, where the number of sublevels on any level equals the energy level number.

Orbital

A region of space that has a $90\%$ probability of containing the electron, capable of holding a maximum of two electrons with opposite spins.