Physical characteristics that are quantized—such as energy, charge, and angular momentum—are of such importance that names and symbols are given to them. The values of quantized entities are expressed in terms of quantum numbers, and the rules governing them are of the utmost importance in determining what nature is and does. This section covers some of the more important quantum numbers and rules—all of which apply in chemistry, material science, and far beyond the realm of atomic physics, where they were first discovered. Once again, we see how physics makes discoveries which enable other fields to grow.

The energy states of bound systems are quantized, because the particle wavelength can fit into the bounds of the system in only certain ways. This was elaborated for the hydrogen atom, for which the allowed energies are expressed as En∝1/n2En∝1/n2, where n=1, 2, 3, …n=1, 2, 3, …. We define
nn to be the principal quantum number that labels the basic states of a system. The lowest-energy state has
n=1n=1, the first excited state has
n=2n=2, and so on. Thus the allowed values for the principal quantum number are

This is more than just a numbering scheme, since the energy of the system, such as the hydrogen atom, can be expressed as some function of nn size 12{n} {}, as can other characteristics (such as the orbital radii of the hydrogen atom).

The fact that the magnitude of angular momentum is quantized was first recognized by Bohr in relation to the hydrogen atom; it is now known to be true in general. With the development of quantum mechanics, it was found that the magnitude of angular momentum LL size 12{L} {} can have only the values

where ll size 12{l} {} is defined to be the angular momentum quantum number. The rule for ll size 12{l} {} in atoms is given in the parentheses. Given nn size 12{n} {}, the value of ll size 12{l} {} can be any integer from zero up to n−1n−1 size 12{n – 1} {}. For example, if n=4n=4 size 12{n=4} {}, then ll size 12{l} {} can be 0, 1, 2, or 3.

Note that for n=1n=1 size 12{n=1} {}, ll size 12{l} {} can only be zero. This means that the ground-state angular momentum for hydrogen is actually zero, not
h/2πh/2π as Bohr proposed. The picture of circular orbits is not valid, because there would be angular momentum for any circular orbit. A more valid picture is the cloud of probability shown for the ground state of hydrogen in [link]. The electron actually spends time in and near the nucleus. The reason the electron does not remain in the nucleus is related to Heisenberg’s uncertainty principle—the electron’s energy would have to be much too large to be confined to the small space of the nucleus. Now the first excited state of hydrogen has n=2n=2 size 12{n=2} {}, so that ll size 12{l} {} can be either 0 or 1, according to the rule in L=ll+1h2πL=ll+1h2π size 12{L= sqrt {l left (l+1 right )} { {h} over {2π} } } {} . Similarly, for n=3n=3 size 12{n=3} {}, ll size 12{l} {} can be 0, 1, or 2. It is often most convenient to state the value of ll size 12{l} {}, a simple integer, rather than calculating the value of LL size 12{L} {} from L=ll+1h2πL=ll+1h2π size 12{L= sqrt {l left (l+1 right )} { {h} over {2π} } } {}. For example, for l=2l=2 size 12{l=2} {}, we see that

It is much simpler to state l=2l=2 size 12{l=2} {}.

As recognized in the Zeeman effect, the direction of angular momentum is quantized. We now know this is true in all circumstances. It is found that the component of angular momentum along one direction in space, usually called the zz size 12{z} {}-axis, can have only certain values of LzLz size 12{L rSub { size 8{z} } } {}. The direction in space must be related to something physical, such as the direction of the magnetic field at that location. This is an aspect of relativity. Direction has no meaning if there is nothing that varies with direction, as does magnetic force. The allowed values of LzLz size 12{L rSub { size 8{z} } } {} are

where LzLz size 12{L rSub { size 8{z} } } {} is the zz size 12{z} {}-component of the angular momentum and mlml size 12{m rSub { size 8{l} } } {} is the angular momentum projection quantum number. The rule in parentheses for the values of mlml size 12{m rSub { size 8{l} } } {} is that it can range from −l−l size 12{ – l} {} to ll size 12{l} {} in steps of one. For example, if l=2l=2 size 12{l=2} {}, then mlml size 12{m rSub { size 8{l} } } {} can have the five values –2, –1, 0, 1, and 2. Each mlml size 12{m rSub { size 8{l} } } {} corresponds to a different energy in the presence of a magnetic field, so that they are related to the splitting of spectral lines into discrete parts, as discussed in the preceding section. If the zz size 12{z} {}-component of angular momentum can have only certain values, then the angular momentum can have only certain directions, as illustrated in [link].

Intrinsic Spin Angular Momentum Is Quantized in Magnitude and Direction

There are two more quantum numbers of immediate concern. Both were first discovered for electrons in conjunction with fine structure in atomic spectra. It is now well established that electrons and other fundamental particles have intrinsic spin, roughly analogous to a planet spinning on its axis. This spin is a fundamental characteristic of particles, and only one magnitude of intrinsic spin is allowed for a given type of particle. Intrinsic angular momentum is quantized independently of orbital angular momentum. Additionally, the direction of the spin is also quantized. It has been found that the magnitude of the intrinsic (internal) spin angular momentum, SS size 12{S} {}, of an electron is given by

S=ss+1h2π(s=1/2 for electrons),S=ss+1h2π(s=1/2 for electrons), size 12{s=1/2} {}

where ss size 12{s} {} is defined to be the spin quantum number. This is very similar to the quantization of LL size 12{L} {} given in L=ll+1h2πL=ll+1h2π size 12{L= sqrt {l left (l+1 right )} { {h} over {2π} } } {}, except that the only value allowed for ss size 12{s} {} for electrons is 1/2.

The direction of intrinsic spin is quantized, just as is the direction of orbital angular momentum. The direction of spin angular momentum along one direction in space, again called the zz size 12{z} {}-axis, can have only the values

Sz=msh2πms=−12,+12Sz=msh2π size 12{S rSub { size 8{z} } =m rSub { size 8{s} } { {h} over {2π} } } {}ms=−12,+12 size 12{ left (m rSub { size 8{s} } = – { {1} over {2} } , + { {1} over {2} } right )} {}

for electrons. SzSz size 12{S rSub { size 8{z} } } {} is the zz size 12{z} {}-component of spin angular momentum and msms size 12{S rSub { size 8{z} } } {} is the spin projection quantum number. For electrons, ss size 12{s} {} can only be 1/2, and msms size 12{m rSub { size 8{s} } } {} can be either +1/2 or –1/2. Spin projection ms=+1/2ms=+1/2 size 12{m rSub { size 8{s} } “=+”1/2} {} is referred to as spin up, whereas ms=−1/2ms=−1/2 size 12{m rSub { size 8{s} } = – 1/2} {} is called spin down. These are illustrated in [link].

Intrinsic Spin

In later chapters, we will see that intrinsic spin is a characteristic of all subatomic particles. For some particles ss size 12{s} {} is half-integral, whereas for others ss size 12{s} {} is integral—there are crucial differences between half-integral spin particles and integral spin particles. Protons and neutrons, like electrons, have s=1/2s=1/2 size 12{s=1/2} {}, whereas photons have s=1s=1 size 12{s=1} {}, and other particles called pions have s=0s=0 size 12{s=0} {}, and so on.

To summarize, the state of a system, such as the precise nature of an electron in an atom, is determined by its particular quantum numbers. These are expressed in the form n, l,ml,msn, l,ml,ms —see [link] For electrons in atoms, the principal quantum number can have the values n=1, 2, 3, …n=1, 2, 3, …. Once nn is known, the values of the angular momentum quantum number are limited to l=1, 2, 3, …,n−1l=1, 2, 3, …,n−1. For a given value of ll, the angular momentum projection quantum number can have only the values ml=−l,−l+1, …,−1, 0, 1, …,l−1,lml=−l,−l+1, …,−1, 0, 1, …,l−1,l. Electron spin is independent of n, l,n, l, and mlml, always having s=1/2s=1/2. The spin projection quantum number can have two values, ms=1/2 or −1/2ms=1/2 or −1/2.

Atomic Quantum Numbers

Name	Symbol	Allowed values
Principal quantum number	nn	1, 2, 3, …1, 2, 3, …
Angular momentum	ll	0, 1, 2, …n−10, 1, 2, …n−1
Angular momentum projection	mlml	−l,−l+1, …,−1, 0, 1, …,l−1,l(or0, ±1, ±2, …,±l)−l,−l+1, …,−1, 0, 1, …,l−1,l(or0, ±1, ±2, …,±l)
Spin1	ss	1/2( electrons ) 1/2 ( electrons )
Spin projection	msms	−1/2, + 1/2 − 1/2, + 1/2

[link] shows several hydrogen states corresponding to different sets of quantum numbers. Note that these clouds of probability are the locations of electrons as determined by making repeated measurements—each measurement finds the electron in a definite location, with a greater chance of finding the electron in some places rather than others. With repeated measurements, the pattern of probability shown in the figure emerges. The clouds of probability do not look like nor do they correspond to classical orbits. The uncertainty principle actually prevents us and nature from knowing how the electron gets from one place to another, and so an orbit really does not exist as such. Nature on a small scale is again much different from that on the large scale.

We will see that the quantum numbers discussed in this section are valid for a broad range of particles and other systems, such as nuclei. Some quantum numbers, such as intrinsic spin, are related to fundamental classifications of subatomic particles, and they obey laws that will give us further insight into the substructure of matter and its interactions.