Electron Configuration and Orbital Filling

For a given atom or ion, the pattern by which orbitals are filled and the number of electrons within
each principal level and subshell are designated by an electron configuration. In electron
configuration notation, the first number denotes the principal energy level, the latter designates the
subshell, and the superscript gives the number of electrons in that subshell. For example, 2p^4
indicates that there are four electrons in the second (p) subshell of the second principal energy level.

When writing the electron configuration of an atom, remember the order in which subshells are
filled. Subshells are filled from lowest to highest energy, and each subshell will fill completely before
electrons begin to enter the next one. The (n + l) rule is used to rank subshells by increasing energy.
This rule states that the lower the values of the first and second quantum numbers, the lower the
energy of the subshell. If two subshells possess the same (n + l) value, the subshell with the lower n
value has a lower energy and will fill first. The order in which the subshells fill is shown in the
following chart, which is arranged so that it is easily remembered: One simply lists the subshells in
order, starting each shell with a new line. The order of filling them is found by crossing them with
diagonal arrows.

Example: Which will fill first, the 3d subshell or the 4s subshell?
Solution: For 3d, n = 3 and l = 2, so (n + l) = 5. For 4s, n = 4 and l = 0, so (n + l) = 4. Therefore, the 4s
subshell has lower energy and will fill first. This can also be determined from the chart
by examination.