The combination of energy and orbital angular momentum together determine the orbital or shell in which the electron resides. We may write an electron orbital as 1s, 2s, 2p, 3d, etc. where the letter here is representative of the azimuthal quantum number $\ell$. The letters are historic relics, but persist anyway. (See Table 1.)
| $\ell$ | Letter | Number of Possible Electrons | Name | Shape |
| 0 | s | 2 | sharp | sphere |
| 1 | p | 6 | principal | two dumbbells |
| 2 | d | 10 | diffuse | four dumbbells |
| 3 | f | 14 | fundamental | eight dumbbells |
| 4 | g | 18 | - | - |
| 5 | h | 22 | - | - |
| Table 1: The letter correspondence for orbital angular momentum quantum number $\ell$. | ||||
This notation is sometimes used to present the configuration of an atom, or the number of electrons in each orbital of an atom. In such a case, the form is $n\ell^x$, where $n$ is the primary (radial) quantum number and $x$ is the number of electrons in that state. For example, boron's ground state configuration is 1s22s22p. There are five electrons with two in the $n=1$, $\ell =0$ state, two in the $n=2$, $\ell =0$ state, and one in the $n=2$, $\ell =1$ state. (Exponents of value 1 are often omitted.) If we want to include more information, we can use another form of spectroscopic notation called the term symbol which has the form
$\textrm{n}^{2\textrm{s}+1}\textrm{L}_\textrm{j}$
where $n$is again the principal quantum number, $s$ is the spin quantum number, $L$ is the (now capitalized) letter corresponding the orbital quantum number $\ell$, and $j$ is the total angular momentum $\left( \overrightarrow j = \overrightarrow s + \overrightarrow \ell\right)$. The $n$ term is considered optional and is often omitted. When transitioning from one state to another, the following selection rules must be obeyed:
$\Delta s = 0$
$\Delta \ell = \pm 1$
$\Delta j = 0^* \pm 1$
$\Delta m_s = 0$
$\Delta m_j = 0\pm 1$
where the transition from $j=0 \rightarrow j=0$ is not allowed, and where $m_s$ and $m_j$ are the z-projections of the spin and total angular momentum, respectively. These rules are a manifestation of the conservation of angular momentum. Consider that when an electron jumps from one energy level to another, it does so by emitting or absorbing a photon. That photon carries with it one unit of angular momentum $\hbar$. If the spin does not change, then that change in angular momentum must be accounted for by an appropriate change in the orbital angular momentum.