In chemistry classes in primary school we learned that atoms "want" to reach noble gas configuration because it have low energy, so atoms on the left of the periodic table are willing to give away electrons to reach that state, while atoms on the right of it would like to take electrons to reach it.
Following the Mandelung rule of filling electron orbitals we reach noble gases when the s and p orbitals of the outer shell fill up.
What's the physical reason that filling up these two types of orbitals on the outer shell causes such a stable and low energy configuration?
Also why don't just filling up an s orbital on the outer shell enough to make a noble gas? Why does it works for Helium both not any atom above?
Best Answer
Indeed, this is a very good question and far from obvious.
From a classical point of view one can only argue using charge distribution and atom radius/ion radius. The more spherical the charge distribution in the atom, the lower its energy in an electric potential, because the more spherically it is, the more neutral the atom in respect to the potential. Remember that the quantum number n represents the shell in c.m.! So whenever we fill up a shell, the charge distribution is most symmetric.
In respect to a quantum mechanical view the arguments are similar. The only spherical symmetric orbital is the s-orbital, thus again, from a classical point of view, the most stable. Also the probability that the electron is located at the nucleus is non zero only for the s-orbital. The nearer the electron to the nucleus, the more stable from the view of the electric potential. As we increase the orbital-angular momentum quantum number l, the average distance of the electron to the nucleus increases.
When reading an organic chemistry textbook which also treats the quantum chemistry behind it, arguments for reactivity are very similar.