The electron configuration

Filling the orbitals: the rules of the game

The electron configuration describes how the electrons of an atom are distributed among the orbitals. It obeys three simple rules, which combine everything we have seen.

Rule 1: from lowest energy upward (Aufbau)

Electrons first occupy the orbitals of lowest energy, then fill the higher levels progressively. Subshells are noted by n followed by the letter (1s, 2s, 2p, 3s...), with the number of electrons as a superscript.

   ordre de remplissage (energie croissante) :

   1s -> 2s -> 2p -> 3s -> 3p -> 4s -> 3d -> ...
   (bas)                                    (haut)

Rule 2: the Pauli principle

Each orbital holds at most two electrons, of opposite spins. So: s -> 2 electrons max, p -> 6, d -> 10.

Rule 3: Hund's rule

Within a same subshell (for example the three p orbitals), electrons first occupy separate orbitals, with parallel spins, before pairing up. They "avoid" ending up two together as long as there are empty boxes remaining.

   Remplissage de 2p avec 3 electrons :

   CORRECT (Hund) :   [↑] [↑] [↑]      (chacun dans sa case, spins //)
   INCORRECT      :   [↑↓][↑] [ ]      (on apparie trop tot)

A complete example: oxygen (8 electrons)

   1s : [↑↓]                    -> 2 electrons  (couche pleine)
   2s : [↑↓]                    -> 2 electrons
   2p : [↑↓][↑ ][↑ ]            -> 4 electrons  (Hund : on separe d'abord)

   configuration :  1s^2  2s^2  2p^4
   total : 2 + 2 + 4 = 8 electrons  ✔

We read "one-s-two, two-s-two, two-p-four". The sum of the exponents (2+2+4) must give back the total number of electrons, here 8 for oxygen.

The link with the periodic table

The configuration explains directly the structure of the periodic table:

   les elements d'une meme COLONNE ont la meme couche externe
        -> memes proprietes chimiques

   ex. couche externe "...2s^1"  (lithium)  et  "...3s^1" (sodium)
        -> tous deux alcalins, tres reactifs, un seul electron a ceder

The row number corresponds to the outer shell being filled; the position within the row, to the block (s, p, d) and to the number of valence electrons. The periodic table is nothing other than a map of electron configurations.

In summary

The electron configuration is built from three rules: fill increasing energies (Aufbau), two electrons max per orbital with opposite spins (Pauli), and occupy separate orbitals with parallel spins before pairing (Hund). It is noted 1s^2 2s^2 2p^4... The resulting outer shell determines the chemical properties and the place of the element in the periodic table — whose entire structure follows from these filling rules.