Quantum theory assigns the electrons surrounding the nucleus to orbitals, which should not be confused with the orbits of the solar system. Each orbital has a characteristic energy and a three‐dimensional shape. An atom in the lowest energy configuration is said to be in its ground state. For this most stable state, the electrons fill the various orbitals from the one of lowest energy to the one of highest energy. Each orbital may be assigned a maximum of two electrons.

The orbitals are described completely by specifying three quantum numbers, but only two are used in this book. The principal quantum number (symbolized n) is a whole number, 1 or greater, that identifies the main energy shell of the orbital, with 1 being closest to the nucleus and each subsequent level farther from the nucleus. The second quantum number, known as the azimuthal quantum number (symbolized ℓ) is a whole number from 0 up to n – 1 that defines the type of orbital within a shell ( n). For historical reasons, the different shapes of orbitals are represented by letters. (See Table 1.)

Because each orbital holds at most two electrons, the maximum number of electrons is twice the number of orbitals for a particular second quantum number. In Table 1, you must know the letters in the second column and the electron capacity in the last column.

A set of orbitals with the same values n and ℓ is called a subshell and is represented by notation like 2 p 5. (See Figure 1.)


Only a few subshells are needed to describe the chemical elements in their ground states. Table 2 lists all the subshells of chemical importance.