Orbitals

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 upward. Each orbital may be assigned a maximum of 2 electrons.

The orbitals are completely described 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 electron shell of the orbital, where the lower digits denote shells of lower energy that are closer to the atomic nucleus. The second 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 2 electrons, the maximum number of electrons is twice the number of orbitals with 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 p⁵. (See Figure 1 .)

Figure 1 A subshell notation.

Figure 2 Filling of the s subshells.

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

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