Electron shell

An electron shell is the set of atomic electron states sharing the same principal quantum number. Its electrons occupy orbitals with different angular momentum and, in multielectron atoms, slightly different energies.

Principal quantum number: An integer quantum number that labels an electron state’s shell and helps determine its size and energy. It is the defining label shared by every electron in a shell.
Subshell: A subdivision of an electron shell containing orbitals with the same principal and azimuthal quantum numbers. Subshells distinguish orbitals that share a shell but differ in angular momentum.
Valence shell: The outermost occupied electron shell of an atom, whose electrons often govern bonding and reactivity. Its occupancy is central to an atom’s chemical behavior.
Bohr model: An early atomic model in which electrons occupy quantized circular orbits around the nucleus. Its fixed orbits resemble shells visually but are not modern electron states.
Atomic orbital: A quantum state of an electron in an atom, specified by quantum numbers and represented by a spatial wavefunction. A shell groups orbitals with the same principal quantum number.
Spin quantum number: A quantum number specifying an electron’s intrinsic angular momentum projection. Spin supplies the second state in each orbital allowed by the Pauli principle.
Periodic table: A tabular arrangement of chemical elements ordered by atomic number and recurring chemical properties. Periods correspond to the filling of successive principal shells.
Electron cloud: A probability distribution describing where an electron may be found around a nucleus. A shell is a quantum-number grouping, not a sharply bounded spatial layer.
Azimuthal quantum number: A quantum number that specifies an orbital’s angular momentum and subshell within an electron shell. Its values divide a shell into s, p, d, and f subshells.
Madelung rule: An empirical rule that estimates orbital filling order using the sums of principal and azimuthal quantum numbers. It gives the familiar sequence in which shells and subshells fill.

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