What are steric interactions?

When first was reading chemistry papers, one piece of jargon jumped out at me:  "steric hindrance", which is an abstruse way of saying that you can't force pieces of molecules (atoms or groups of atoms) to pass through each other.  In physics jargon, they have a "hard core repulsion".  If you want to describe the potential energy of two atoms as you try to squeeze one into the volume of the other, you get a term that blows up very rapidly, like \(1/r^{12}\), where \(r\) is the distance between the nuclei.  Basically, you can do pretty well treating atoms like impenetrable spheres with diameters given by their outer electronic orbitals.  Indeed, Robert Hooke went so far as to infer, from the existence of faceted crystals, that matter is built from effectively impenetrable little spherical atoms.

It's a common thing in popular treatments of physics to point out that atoms are "mostly empty space".  With hydrogen, for example, if you said that the proton was the size of a pea, then the 1s orbital (describing the spatial probability distribution for finding the point-like electron) would be around 250 m in radius.  So, if atoms are such big, puffy objects, then why can't two atoms overlap in real space?  It's not just the electrostatic repulsion, since each atom is overall neutral.

The answer is (once again) the Pauli exclusion principle (PEP) and the fact that electrons obey Fermi statistics.  Sometimes the PEP is stated in a mathematically formal way that can obscure its profound consequences.  For our purposes, the bottom line is:  It is apparently a fundamental property of the universe that you can't stick two identical fermions (including having the same spin) in the same quantum state.    At the risk of getting technical, this can mean a particular atomic orbital, or more generally it can be argued to mean the same little "cell" of volume \(h^{3}\) in r-p phase space.  It just can't happen. 

If you try to force it, what happens instead?  In practice, to get two carbon atoms, say, to overlap in real space, you would have to make the electrons in one of the atoms leave their ordinary orbitals and make transitions to states with higher kinetic energies.  That energy has to come from somewhere - you have to do work and supply that energy to squeeze two atoms into the volume of one.  Books have been written about this.

Leaving aside for a moment the question of why rigid solids are rigid, it's pretty neat to realize that the physics principle that keeps you from falling through your chair or the floor is really the same principle that holds up white dwarf stars.