Quantum numbers describe values of conserved quantities in the dynamics of the quantum system. Perhaps the most peculiar aspect of quantum mechanics is the quantization of observable quantities, since quantum numbers are discrete sets of integers or half-integers. This is distinguished from classical mechanics where the values can range continuously. Quantum numbers often describe specifically the energies of electrons in atoms, but other possibilities include angular momentum, spin, etc. Any quantum system can have one or more quantum numbers; it is thus difficult to list all possible quantum numbers.
How many quantum numbers?
The question of how many quantum numbers are needed to describe any given system has no universal answer, hence for each system, one must find the answer for a full analysis of the system. A quantized system requires at least one quantum number. The dynamics of any quantum system are described by a quantum Hamiltonian, H. There is one quantum number of the system corresponding to the energy, i.e., the eigenvalue of the Hamiltonian. There is also one quantum number for each operator O that commutes with the Hamiltonian (i.e. satisfies the relation HO = OH). These are all the quantum numbers that the system can have. Note that the operators O defining the quantum numbers should be independent of each other. Often, there is more than one way to choose a set of independent operators. Consequently, in different situations different sets of quantum numbers may be used for the description of the same system.
Spatial and angular momentum numbers
To completely describe an electron in an atom, four quantum numbers are needed: energy, angular momentum, magnetic moment, and spin.
Many different models have been proposed throughout the history of quantum mechanics, but the most prominent system of nomenclature spawned from the Hund-Mulliken molecular orbital theory of Friedrich Hund, Robert S. Mulliken, and contributions from Schrödinger, Slater and John Lennard-Jones. This system of nomenclature incorporated Bohr energy levels, Hund-Mulliken orbital theory, and observations on electron spin based on spectroscopy and Hund's rules.
This model describes electrons using four quantum numbers, n, ℓ, mℓ, ms, given below. It is also the common nomenclature in the classical description of nuclear particle states (e.g. protons and neutrons). Molecular orbitals require different quantum numbers, because the Hamiltonian and its symmetries are quite different.
- The principal quantum number: n The first describes the electron shell, or energy level, of an atom. The value of n ranges from 1 to the shell containing the outermost electron of that atom, i.e.
- n = 1, 2, ... .
- The azimuthal quantum number: ℓ The second (also known as the angular quantum number or orbital quantum number) describes the subshell, and gives the magnitude of the orbital angular momentum through the relation
- L2 = ħ2 ℓ (ℓ + 1).
- ℓ = 0, 1, 2,..., n − 1.
- The magnetic quantum number: mℓ The third describes the specific orbital (or "cloud") within that subshell, and yields the projection of the orbital angular momentum along a specified axis:
- Lz = mℓ ħ.
- The spin projection quantum number: ms The fourth describes the spin (intrinsic angular momentum) of the electron within that orbital, and gives the projection of the spin angular momentum S along the specified axis:
- Sz = ms ħ.
- ms = −s, −s + 1, −s + 2,...,s − 2, s − 1, s.
Note that, since atoms and electrons are in a state of constant motion, there is no universal fixed value for mℓ and ms values. Therefore, the mℓ and ms values are defined somewhat arbitrarily. The only requirement is that the naming schematic used within a particular set of calculations or descriptions must be consistent (e.g. the orbital occupied by the first electron in a p subshell could be described as mℓ = −1 or mℓ = 0, or mℓ = 1, but the mℓ value of the other electron in that orbital must be the same, and the mℓ assigned to electrons in other orbitals must be different).
These rules are summarized as follows:
Name Symbol Orbital meaning Range of values Value examples principal quantum number n shell 1 ≤ n n = 1, 2, 3, … azimuthal quantum number (angular momentum) ℓ subshell (s orbital is listed as 0, p orbital as 1 etc.) 0 ≤ ℓ ≤ n − 1 for n = 3:
ℓ = 0, 1, 2 (s, p, d)
magnetic quantum number, (projection of angular momentum) mℓ energy shift (orientation of the subshell's shape) −ℓ ≤ mℓ ≤ ℓ for ℓ = 2:
mℓ = −2, −1, 0, 1, 2
spin projection quantum number ms spin of the electron (−½ = "spin down", ½ = "spin up") −s ≤ ms ≤ s for an electron s = ½,
so ms = −½, ½
Example: The quantum numbers used to refer to the outermost valence electrons of the Carbon (C) atom, which are located in the 2p atomic orbital, are; n = 2 (2nd electron shell), ℓ = 1 (p orbital subshell), mℓ = 1, 0 or −1, ms = ½ (parallel spins).
Results from spectroscopy indicated that up to two electrons can occupy a single orbital. However two electrons can never have the same exact quantum state nor the same set of quantum numbers according to Hund's rules, which addresses the Pauli exclusion principle. A fourth quantum number with two possible values was added as an ad hoc assumption to resolve the conflict; this supposition could later be explained in detail by relativistic quantum mechanics and from the results of the renowned Stern-Gerlach experiment.
Total angular momenta numbers
Total momentum of a particle
When one takes the spin-orbit interaction into consideration, the L and S operators no longer commute with the Hamiltonian, and their eigenvalues therefore change over time. Thus another set of quantum numbers should be used. This set includes
- The total angular momentum quantum number:
- j = |ℓ ± s|
- J2 = ħ2 j (j + 1).
- The projection of the total angular momentum along a specified axis:
- mj = −j, −j + 1, −j + 2,...,j − 2, j − 1, j
- mj = mℓ + ms and |mℓ + ms| ≤ j.
- Parity This is the eigenvalue under reflection, and is positive (+1) for states which came from even ℓ and negative (−1) for states which came from odd ℓ. The former is also known as even parity and the latter as odd parity, and is given by
- P = (−1)ℓ.
For example, consider the following eight states, defined by their quantum numbers:
n ℓ mℓ ms ℓ + s ℓ - s ml + ms #1. 2 1 1 +1/2 3/2 1/2 3/2 #2. 2 1 1 -1/2 3/2 1/2 1/2 #3. 2 1 0 +1/2 3/2 1/2 1/2 #4. 2 1 0 -1/2 3/2 1/2 -1/2 #5. 2 1 -1 +1/2 3/2 1/2 -1/2 #6. 2 1 -1 -1/2 3/2 1/2 -3/2 #7. 2 0 0 +1/2 1/2 -1/2 1/2 #8. 2 0 0 -1/2 1/2 -1/2 -1/2
The quantum states in the system can be described as linear combination of these eight states. However, in the presence of spin-orbit interaction, if one wants to describe the same system by eight states which are eigenvectors of the Hamiltonian (i.e. each represents a state which does not mix with others over time), we should consider the following eight states:
j = 3/2, mj = 3/2, odd parity (coming from state (1) above) j = 3/2, mj = 1/2, odd parity (coming from states (2) and (3) above) j = 3/2, mj = -1/2, odd parity (coming from states (4) and (5) above) j = 3/2, mj = -3/2, odd parity (coming from state (6) above) j = 1/2, mj = 1/2, odd parity (coming from states (2) and (3) above) j = 1/2, mj = -1/2, odd parity (coming from states (4) and (5) above) j = 1/2, mj = 1/2, even parity (coming from state (7) above) j = 1/2, mj = -1/2, even parity (coming from state (8) above)
Nuclear angular momentum quantum numbers
In nuclei, the entire assembly of protons and neutrons (nucleons) has a resultant angular momentum due to the angular momenta of each nucleon, usually denoted I. If the total angular momentum of a neutron is jn = ℓ + s and for a proton is jp = ℓ + s (where s for protons and neutrons happens to be ½ again) then the nuclear angular momentum quantum numbers I are given by:
- I = |jn − jp|, |jn − jp| + 1, |jn − jp| + 2,..., (jn + jp) − 2, (jn + jp) − 1, (jn + jp)
H11 I = (1/2)+ C69 I = (3/2)− Na1120 I = 2+ H12 I = 1+ C610 I = 0+ Na1121 I = (3/2)+ H13 I = (1/2)+ C611 I = (3/2)− Na1122 I = 3+ C612 I = 0+ Na1123 I = (3/2)+ C613 I = (1/2)− Na1124 I = 4+ C614 I = 0+ Na1125 I = (5/2)+ C615 I = (1/2)+ Na1126 I = 3+
The reason for the unusual fluctuations in I, even by differences of just one nucleon, are due to the odd/even numbers of protons and neutrons - pairs of nucleons have a total angular momentum of zero (just like electrons in orbitals), leaving an odd/even numbers of unpaired nucleons. The property of nuclear spin is an important factor for the operation of NMR spectroscopy in organic chemistry, and MRI in nuclear medicine, due to the nuclear magnetic moment interacting with an external magnetic field.
Elementary particles contain many quantum numbers which are usually said to be intrinsic to them. However, it should be understood that the elementary particles are quantum states of the standard model of particle physics, and hence the quantum numbers of these particles bear the same relation to the Hamiltonian of this model as the quantum numbers of the Bohr atom does to its Hamiltonian. In other words, each quantum number denotes a symmetry of the problem. It is more useful in quantum field theory to distinguish between spacetime and internal symmetries.
Typical quantum numbers related to spacetime symmetries are spin (related to rotational symmetry), the parity, C-parity and T-parity (related to the Poincaré symmetry of spacetime). Typical internal symmetries are lepton number and baryon number or the electric charge. (For a full list of quantum numbers of this kind see the article on flavour.)
A minor but often confusing point is as follows: most conserved quantum numbers are additive, so in an elementary particle reaction, the sum of the quantum numbers should be the same before and after the reaction. However, some, usually called a parity, are multiplicative; i.e., their product is conserved. All multiplicative quantum numbers belong to a symmetry (like parity) in which applying the symmetry transformation twice is equivalent to doing nothing. These are all examples of an abstract group called Z2.
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