Definitions

# Azimuthal quantum number

The Azimuthal quantum number (or orbital angular momentum quantum number, second quantum number) symbolized as l (lower-case L) is a quantum number for an atomic orbital that determines its orbital angular momentum. The azimuthal quantum number is the second of a set of quantum numbers (the principal quantum number, following spectroscopic notation, the azimuthal quantum number, the magnetic quantum number, and the spin quantum number) which describe the unique quantum state of an electron and is designated by the letter l.

## Derivation

There is a set of quantum numbers associated with the energy states of the electrons of an atom. The four quantum numbers n, l, ml, and ms specify the complete and unique quantum state of a single electron in an atom called its wavefunction or orbital. The wavefunction of the Schrödinger wave equation reduces to the three equations that when solved lead to the first three quantum numbers. Therefore, the equations for the first three quantum numbers are all interrelated. The azimuthal quantum number arose in the solution of the polar part of the wave equation as shown below. In addition to the understanding of this concept of the azimuth, one may also find it necessary to review or learn more about spherical coordinate systems, and or other alternative mathematical coordinate systems other than the cartesian system. It is known that the spherical coordinate system works best with spherical models, the cylindrical system with cylinders, the cartesian with general areas, etc.. The concept of the azimuth and how it used to explain electrons may be more understandable after such a review.

An atomic electron's angular momentum, L, which is related to its quantum number $mathbf\left\{\right\}l$ is described by the following equation:

$mathbf\left\{L^2boldsymbol\left\{psi\right\}\right\} = hbar^2\left\{l\left(l+1\right)\right\}boldsymbol\left\{psi\right\}$

where $hbar = h/2pi$ is the reduced Planck's constant, also called Dirac's constant, $mathbf\left\{L^2\right\}$ is the orbital angular momentum operator and $boldsymbol\left\{psi\right\}$ is the wavefunction of the electron. While many introductory text books on quantum mechanics will refer to L by itself, L has no real meaning except in its use as the angular momentum operator. When referring to angular momentum, it is best to simply use the quantum number $l,$.

The energy of any wave is the frequency multiplied by Planck's constant. This causes the wave to display particle-like packets of energy called quanta. To show each of the quantum numbers in the quantum state, the formulae for each quantum number include Planck's reduced constant which only allows particular or discrete or quantized energy levels.

This behavior manifests itself as the "shape" of the orbital.

Electron shells have distinctive shapes denoted by letters. In the illustration, the letters s, p, and d describe the shape of the atomic orbital.

Their wavefunctions take the form of spherical harmonics, and so are described by Legendre polynomials. The various orbitals relating to different values of l are sometimes called sub-shells, and (mainly for historical reasons) are referred to by letters, as follows:

$l$ Letter Max electrons Shape Name
0 s 2 sphere sharp
1 p 6 two dumbbells principal
2 d 10 four dumbbells diffuse
3 f 14 eight dumbbells fundamental
4 g 18
5 h 22
6 i 26

A mnemonic for the order of the "shells" is some poor dumb fool. Another mnemonic for the order of the "shells" is silly professors dance funny. The letters after the F subshell just follow F in alphabetical order.

Each of the different angular momentum states can take 2(2l+1) electrons. This is because the third quantum number ml (which can be thought of loosely as the quantized projection of the angular momentum vector on the z-axis) runs from −l to l in integer units, and so there are 2l+1 possible states. Each distinct nlml orbital can be occupied by two electrons with opposing spins (given by the quantum number ms), giving 2(2l+1) electrons overall. Orbitals with higher l than given in the table are perfectly permissible, but these values cover all atoms so far discovered.

For a given value of the principal quantum number, n, the possible values of l range from 0 to n−1; therefore, the n=1 shell only possesses an s subshell and can only take 2 electrons, the n=2 shell possesses an s and a p subshell and can take 8 electrons overall, the n=3 shell possesses s, p and d subshells and has a maximum of 18 electrons, and so on (generally speaking, the maximum number of electrons in the nth energy level is 2n2).

The angular momentum quantum number, l, governs the number of planar nodes going through the nucleus. A planar node can be described in an electromagnetic wave as the midpoint between crest and trough, which has zero magnitude. In an s orbital, no nodes go through the nucleus, therefore the corresponding azimuthal quantum number l takes the value of zero. In a p orbital, one node traverses the nucleus and therefore l has the value 1.

Depending on the value of n, the principal quantum number, there is an angular momentum quantum number l and the following series. The wavelengths listed are for a hydrogen atom:

n = 1, l = 0, Lyman series (ultraviolet)
n = 2, l = ħ, Balmer series (visible) Wavelength vary from 400 to 700 nm
n = 3, l = 2ħ, Ritz-Paschen series (short wave infrared)
n = 4, l = 3ħ, Pfund series (long wave infrared)

## Addition of quantized angular momenta

Given a quantized total angular momentum $overrightarrow\left\{j\right\}$ which is the sum of two individual quantized angular momenta $overrightarrow\left\{l_1\right\}$ and $overrightarrow\left\{l_2\right\}$,

$overrightarrow\left\{j\right\} = overrightarrow\left\{l_1\right\} + overrightarrow\left\{l_2\right\}$

the quantum number $j$ associated with its magnitude can range from $|l_1 - l_2|$ to $l_1 + l_2$ in integer steps where $l_1$ and $l_2$ are quantum numbers corresponding to the magnitudes of the individual angular momenta.

### Total angular momentum of an electron in the atom

Due to the spin-orbit interaction in the atom, the orbital angular momentum no longer commutes with the Hamiltonian, nor does the spin. These therefore change over time. However the total angular momentum J does commute with the Hamiltonian and so is constant. J is defined through

$overrightarrow\left\{J\right\} = overrightarrow\left\{L\right\} + overrightarrow\left\{S\right\}$
L being the orbital angular momentum and S the spin. The total angular momentum satisfies the same commutation relations as angular momentum, namely
$\left[J_i, J_j \right] = i hbar epsilon_\left\{ijk\right\} J_k$
from which follows
$left\left[J_i, J^2 right\right] = 0$
where $J_\left\{i,j\right\}$ stand for $J_x$, $J_y$ and $J_z$.

The quantum numbers describing the system, which are constant over time, are now j and $m_j$, defined through the action of J on the wavefunction $psi$

$mathbf\left\{J^2boldsymbol\left\{psi\right\}\right\} = hbar^2\left\{j\left(j+1\right)\right\}boldsymbol\left\{psi\right\}$
$mathbf\left\{J_zboldsymbol\left\{psi\right\}\right\} = hbar\left\{m_j\right\}boldsymbol\left\{psi\right\}$

So that j is related to the norm of the total angular momentum and $m_j$ to its projection along a specified axis.

As with any angular momentum in quantum mechanics, the projection of J along other axes cannot be co-defined with $J_z$, because they do not commute.

#### Relation between new and old quantum numbers

j and mj, together with the parity of the quantum state, replace the three quantum numbers l, ml and ms (the projection of the spin along the specified axis). The former quantum numbers can be related to the latter.

Furthermore, the eigenvectors of j, mj and parity, which are also eigenvectors of the Hamiltonian, are linear combinations of the eigenvectors of l, ml and ms.

## History

The azimuthal quantum number was carried over from the Bohr model of the atom. The Bohr model was derived from spectroscopic analysis of the atom in combination with the Rutherford atomic model. The lowest quantum level was found to have an angular momentum of zero. To simplify the mathematics, orbits were considered as oscillating charges in one dimension and so described as "pendulum" orbits. In three-dimensions the orbit becomes spherical without any nodes crossing the nucleus, similar to a jump rope that oscillates in one large circle.