Electromagnetic force and resistance to displacement of atoms.

[PrincePhoenix]

It is written over here http://www.particleadventure.org/electromagnetism.html that,
"...the force that keeps you from falling through the floor is the electromagnetic force which causes the atoms making up the matter in your feet and the floor to resist being displaced."
Can anyone explain how the electromagnetic force causes the atoms resist being displaced?
And how does it cause friction?

 

[Bob_for_short]

It is easy. Atoms interact with some sort of a potential force U(r) (a complex result of quantum mechnaical effects).
At small displacements from the equilibrium positions the potentials U(r_i) can be represented as quadratic ones => the forces are elastic (F=-kx) and returning. So any external force makes atoms displace until their new equilibrium positions where the elastic reaction force compensates the external force. QM is very important here - for atom stability.