Atomic Structure - Discovery and Models

Vocabulary practice covering atomic structure, discovery of subatomic particles, nuclear size calculations, isotopes/isobars, wave characteristics, Planck's Quantum Theory, the Photoelectric Effect, and Bohr's Atomic Model.

Cathode Rays

Invisible rays discovered by Julius Plucker in 1859 that move from the negative electrode to the positive electrode in a discharge tube at low pressure ($10^{-4}\,atm$) and high voltage.

Electron

The basic negatively charged constituent of all atoms, possessing a charge-to-mass ratio ($e/m$) of $1.76 \times 10^{11}\,C\,kg^{-1}$.

Anode Rays (Canal Rays)

Positively charged rays discovered by Goldstein in 1886 that originate from the anode and pass through a perforated cathode.

Proton

The lightest positive particle obtained from hydrogen gas in a discharge tube, carrying a charge of $+1.6022 \times 10^{-19}\,Coulombs$.

Neutron

An electrically neutral particle discovered by James Chadwick in 1932 by bombarding a thin sheet of Beryllium with $\alpha$-particles.

Fundamental Particles

The three primary particles that make up atoms: electrons, protons, and neutrons (present in all atoms except hydrogen, which lacks neutrons).

Thomson's Model of the Atom

Assumes the atom is a uniform sphere of positive charges with electrons embedded in it, maintaining electrical neutrality.

Rutherford's Nuclear Concept

A model where the atom consists of a small, positively charged nucleus at the center carrying almost all the mass, while electrons move in concentric circular orbits around it.

Nucleus

The small, dense region at the center of an atom with a diameter on the order of $10^{-12}$ to $10^{-13}\,cm$, discovered via the $\alpha$-particle scattering experiment.

Atomic Number ($Z$)

The total number of protons present in the nucleus of an atom, also known as the proton number.

Mass Number ($A$)

The sum of the number of protons and neutrons present in the nucleus of an atom ($A = Z + n$).

Nucleons

The collective term for protons and neutrons because they are located within the nucleus.

Isotopes

Atoms of the same element having the same atomic number but different mass numbers, such as Protium ($^{1}H$), Deuterium ($^{2}D$), and Tritium ($^{3}T$).

Isobars

Atoms of different elements that have the same mass number but different atomic numbers, such as $_{18}Ar$, $_{19}K$, and $_{20}Ca$, each with a mass of $40$.

Isotones

Atoms of different elements containing the same number of neutrons.

Isoelectronic Species

Atoms or ions that contain the same number of electrons, such as $O^{2-}$, $F^{-}$, and $Na^{+}$.

Radius of Nucleus Formula

The radius is proportional to the cube root of the mass number: $R = R_0 (A)^{1/3}$, where $R_0$ is approximately $1.1 \times 10^{-13}$ to $1.44 \times 10^{-13}\,cm$.

Electromagnetic Radiations

Oscillating electrical and magnetic fields that are perpendicular to each other and travel at the speed of light ($2.997925 \times 10^8\,m/s$) in a vacuum.

Wavelength ($\lambda$)

The distance between any two consecutive crests or troughs in a wave, often expressed in Angstroms ($\mathring{A}$), meters ($m$), or nanometers ($nm$).

Frequency ($\nu$)

The number of waves passing through a point in one second, expressed in Hertz ($Hz$) or cycles per second ($s^{-1}$).

Wave Number ($\bar{\nu}$)

The number of waves present in $1\,cm$ length, which is equal to the reciprocal of the wavelength: $\bar{\nu} = \frac{1}{\lambda}$.

Black Body

An ideal body that emits and absorbs all frequencies of radiation.

Planck's Quantum Theory

States that atoms and molecules can emit or absorb energy only in discrete quantities called quanta, rather than in a continuous manner.

Quantum of Light (Photon)

A packet or bundle of energy representing the smallest quantity of energy that can be emitted or absorbed as electromagnetic radiation, with energy $E = h\nu$.

Planck's Constant ($h$)

A fundamental physical constant with a value of $6.626 \times 10^{-34}\,J\cdot s$.

One electron volt ($eV$)

The energy gained by an electron when it is accelerated through a potential difference of $1\,volt$, equal to $1.6 \times 10^{-19}\,J$.

Photoelectric Effect

The phenomenon where electrons are ejected from a metal surface when a beam of light with a frequency higher than the threshold frequency strikes it.

Threshold Frequency ($\nu_0$)

The characteristic minimum frequency for a specific metal below which the photoelectric effect is not observed.

Work Function ($W_0$)

The minimum energy required to eject an electron from a metal surface, calculated as $h\nu_0$.

Einstein's Photoelectric Equation

A mathematical representation based on conservation of energy: $h\nu = h\nu_0 + \frac{1}{2} m_e v^2$, where the excess energy becomes kinetic energy for the photoelectron.

Bohr's Stationary Orbits

Definite orbits around the nucleus where an electron can revolve without emitting electromagnetic radiation.

Angular Momentum Quantization (Bohr)

The principle that the angular momentum of an electron in a stationary orbit must be an integral multiple of $\frac{h}{2\pi}$, expressed as $mvr = \frac{nh}{2\pi}$.