Quantum number
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Quantum mechanics |
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In
To fully specify the state of the electron in a hydrogen atom, four quantum numbers are needed. The traditional set of quantum numbers includes the
History
Electronic quantum numbers
In the era of the
As Bohr notes in his subsequent Nobel lecture, the next step was taken by Arnold Sommerfeld in 1915.[4] Sommerfeld's atomic model added a second quantum number and the concept of quantized phase integrals to justify them.[5]: 207 Sommerfeld's model was still essentially two dimensional, modeling the electron as orbiting in a plane; in 1919 he extended his work to three dimensions using 'space quantization' in place of the quantized phase integrals.[6]: 152 Karl Schwarzschild and Sommerfeld's student, Paul Epstein, independently showed that adding third quantum number gave a complete account for the Stark effect results.
A consequence of space quantization was that the electron's orbital interaction with an external magnetic field would be quantized. This seemed to be confirmed when the results of the
The fourth and fifth quantum numbers of the atomic era arose from attempts to understand the Zeeman effect. Like the Stern-Gerlach experiment, the Zeeman effect reflects the interaction of atoms with a magnetic field; in a weak field the experimental results were called "anomalous", they diverged from any theory at the time. Wolfgang Pauli's solution to this issue was to introduce another quantum number taking only two possible values, .[8] This would ultimately become the quantized values of the projection of spin, an intrinsic angular momentum quantum of the electron. In 1927 Ronald Fraser demonstrated that the quantization in the Stern-Gerlach experiment was due to the magnetic moment associated with the electron spin rather than its orbital angular momentum.[7] Pauli's success in developing the arguments for a spin quantum number without relying on classical models set the stage for the development of quantum numbers for elementary particles in the remainder of the 20th century.[8]
Bohr, with his
Nuclear quantum numbers
With successful models of the atom, the attention of physics turned to models of the nucleus. Beginning with Heisenberg's initial model of proton-neutron binding in 1932, Eugene Wigner introduced isospin in 1937, the first 'internal' quantum number unrelated to a symmetry in real space-time.[10]: 45
Connection to symmetry
As quantum mechanics developed, abstraction increased and models based on symmetry and invariance played increasing roles. Two years before his work on the quantum wave equation, Schrödinger applied the symmetry ideas originated by
General properties
Good quantum numbers correspond to
The tally of quantum numbers varies from system to system and has no universal answer. Hence these parameters must be found for each system to be analyzed. A quantized system requires at least one quantum number. The dynamics (i.e. time evolution) of any quantum system are described by a
Electron in a hydrogen-like atom
Four quantum numbers can describe an electron energy level in a hydrogen-like atom completely:
- Principal quantum number (n)
- Azimuthal quantum number (ℓ)
- Magnetic quantum number (mℓ)
- Spin quantum number (ms)
These quantum numbers are also used in the classical description of nuclear particle states (e.g. protons and neutrons).[
Principal quantum number
The principal quantum number describes the electron shell of an electron. The value of n ranges from 1 to the shell containing the outermost electron of that atom, that is[12]
- n = 1, 2, ...
For example, in caesium (Cs), the outermost valence electron is in the shell with energy level 6, so an electron in caesium can have an n value from 1 to 6. The average distance between the electron and the nucleus increases with n.
Azimuthal quantum number
The azimuthal quantum number, also known as the orbital angular momentum quantum number, describes the subshell, and gives the magnitude of the orbital angular momentum through the relation
- L2 = ħ2 ℓ (ℓ + 1).
In chemistry and spectroscopy, ℓ = 0 is called s orbital, ℓ = 1, p orbital, ℓ = 2, d orbital, and ℓ = 3, f orbital.
The value of ℓ ranges from 0 to n − 1, so the first p orbital (ℓ = 1) appears in the second electron shell (n = 2), the first d orbital (ℓ = 2) appears in the third shell (n = 3), and so on:[13]
- ℓ = 0, 1, 2,..., n − 1
A quantum number beginning in n = 3,ℓ = 0, describes an electron in the s orbital of the third electron shell of an atom. In chemistry, this quantum number is very important, since it specifies the shape of an
Magnetic quantum number
The magnetic quantum number describes the specific orbital within the subshell, and yields the projection of the orbital angular momentum along a specified axis:
- Lz = mℓ ħ
The values of mℓ range from −ℓ to ℓ, with integer intervals.[14][page needed]
The s subshell (ℓ = 0) contains only one orbital, and therefore the mℓ of an electron in an s orbital will always be 0. The p subshell (ℓ = 1) contains three orbitals, so the mℓ of an electron in a p orbital will be −1, 0, or 1. The d subshell (ℓ = 2) contains five orbitals, with mℓ values of −2, −1, 0, 1, and 2.
Spin magnetic quantum number
The
- Sz = ms ħ.
In general, the values of ms range from −s to s, where s is the spin quantum number, associated with the magnitude of particle's intrinsic spin angular momentum:[15]
- ms = −s, −s + 1, −s + 2, ..., s − 2, s − 1, s.
An electron state has spin number s = 1/2, consequently ms will be +1/2 ("spin up") or -1/2 "spin down" states. Since electron are
The Aufbau principle and Hund's Rules
A multi-electron atom can be modeled qualitatively as a hydrogen like atom with higher nuclear charge and correspondingly more electrons. The occupation of the electron states in such an atom can be predicted by the Aufbau principle and Hund's empirical rules for the quantum numbers. The Aufbau principle fills orbitals based on their principal and azimuthal quantum numbers (lowest first, with lowest breaking ties; Hund's rule favors unpaired electrons in the outermost orbital). These rules are empirical but they can be related to electron physics.[16]: 10 [17]: 260
Spin-orbit coupled systems
When one takes the
- The total angular momentum quantum number:
- j = |ℓ ± s|
which gives the total angular momentum through the relation
- 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
analogous to the above and satisfies
- mj = mℓ + ms and |mℓ + ms| ≤ j
- Parity
This is the
eigenvalueunder reflection: 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 8 states, defined by their quantum numbers:
n ℓ mℓ ms ℓ + s ℓ − s mℓ + ms (1) 2 1 1 +1/2 3/2 1/23/2 (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
j mj parity 3/2 3/2 odd coming from state (1) above 3/2 1/2 odd coming from states (2) and (3) above 3/2 −1/2 odd coming from states (4) and (5) above 3/2 −3/2 odd coming from state (6) above 1/2 1/2 odd coming from states (2) and (3) above 1/2 −1/2 odd coming from states (4) and (5) above 1/2 1/2 even coming from state (7) above 1/2 −1/2 even coming from state (8) above
Atomic nuclei
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 1/2 again (see note)), 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)
Note: The orbital angular momenta of the nuclear (and atomic) states are all integer multiples of ħ while the intrinsic angular momentum of the neutron and proton are half-integer multiples. It should be immediately apparent that the combination of the intrinsic spins of the nucleons with their orbital motion will always give half-integer values for the total spin, I, of any odd-A nucleus and integer values for any even-A nucleus.
Parity with the number I is used to label nuclear angular momentum states, examples for some isotopes of hydrogen (H), carbon (C), and sodium (Na) are;[20]
- 1
1HI = (1/2)+ 9
6CI = (3/2)− 20
11NaI = 2+ 2
1HI = 1+ 10
6CI = 0+ 21
11NaI = (3/2)+ 3
1HI = (1/2)+ 11
6CI = (3/2)− 22
11NaI = 3+ 12
6CI = 0+ 23
11NaI = (3/2)+ 13
6CI = (1/2)− 24
11NaI = 4+ 14
6CI = 0+ 25
11NaI = (5/2)+ 15
6CI = (1/2)+ 26
11NaI = 3+
The reason for the unusual fluctuations in I, even by differences of just one nucleon, are due to the odd and even numbers of protons and neutrons – pairs of nucleons have a total angular momentum of zero (just like electrons in orbitals), leaving an odd or even number of unpaired nucleons. The property of nuclear spin is an important factor for the operation of
Elementary particles
Typical quantum numbers related to
Multiplicative quantum numbers
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 (involution).
See also
References
- .
- ISBN 978-0-486-26126-3.
- ISSN 0028-0836.
- ^ Niels Bohr – Nobel Lecture. NobelPrize.org. Nobel Prize Outreach AB 2024. Sun. 25 Feb 2024.
- ISBN 978-1-4614-7461-6.
- ISBN 978-0-19-965498-7.
- ^ ISSN 0031-9228.
- ^ ISSN 1355-2198.
- ISBN 978-0-19-965498-7.
- ISBN 978-0-444-87099-5.
- ^ ISBN 978-0-19-956684-6.
- ISBN 0-07-100144-1.[page needed]
- ISBN 0-19-855129-0.[page needed]
- ^ Eisberg & Resnick 1985.
- ISBN 978-0-07-162358-2.[page needed]
- ISBN 0-07-032760-2.
- ISBN 978-0-07-037421-8.
- ISBN 0-19-855129-0.[page needed]
- ^ a b Atkins, P. W. (1977). Molecular Quantum Mechanics Part III: An Introduction to Quantum Chemistry. Vol. 2. Oxford University Press.[ISBN missing][page needed]
- ^ ISBN 978-0-471-80553-3.[page needed]
Further reading
- ISBN 0-19-852011-5.
- ISBN 0-13-805326-X.
- ISBN 0-471-88741-2.
- Eisberg, Robert Martin; ISBN 978-0-471-87373-0 – via Internet Archive.