To determine which subshells are occupied, you must know the number of electrons in the atom. In
the case of uncharged atoms, the number of electrons is equal to the atomic number. If the atom is
charged, the number of electrons is equal to the atomic number plus the extra electrons if the atom
is negative, or the atomic number minus the missing electrons if the atom is positive.
BASIC CONCEPT
Hund’s rule: Put one electron in each orbital in the same subshell first before pairing.In subshells that contain more than one orbital, such as the 2p subshell with its 3 orbitals, the
orbitals will fill according to Hund’s rule. Hund’s rule states that within a given subshell, orbitals are
half-filled so that they each have one electron, all with parallel spins, before any orbital is fully
occupied with two electrons of opposite spins. In other words, electrons would tend to avoid pairing
as much as possible.
Example: What are the written electron configurations for nitrogen (N) and iron (Fe) according to
Hund’s rule?
Solution: Nitrogen has an atomic number of 7, thus its electron configuration is 1s^2 2 s^2 2 p^3.
According to Hund’s rule, the two s orbitals will fill completely, while the three p
orbitals will each contain one electron, all with parallel spins.
Iron has an atomic number of 26, and its 4s subshell fills before the 3d. Using Hund’s rule, the
electron configuration will be:Iron’s electron configuration is written as 1s^2 2 s^2 2 p^6 3 s^2 3 p^6 3 d^6 4 s^2 . Subshells may be listed
either in the order in which they fill (e.g., 4s before 3d) or with subshells of the same principal
quantum number grouped together, as shown here. Both methods are correct.