Question

Although the noble gas elements are monatomic and could not give rise to dipole-dipole forces or hydrogen bonding, these elements still can be liquefied and solidified. Could you explain this?

Answer 1

Because there actually are small dipoles. They just are not permanent ones.

Explanation:

Say you have a long box containing a nominally uniform distribution of nitrogen gas. On average you would expect half the nitrogen molecules to be in either half of the box; but if you could freeze the molecules at any given time and count the number in each half you are unlikely to have it come out exactly even. Just because of the random back and forth motion of the molecules you may get a slight nonuniformity. For a box you can comfortably hold in front of you the nonuniformity, due to this random motion, might be roughly `10^{11}` molecules out of `10^{22}`.

Similarly, although not governed by the same statistical laws, electrons in atoms can undergo random fluctuations within their orbitals. Like the nitrogen molecules in the box, the electrons in a noble gas atom are on average balanced in all directions, but at any one time the balance does not hold exactly because of the random fluctuations. You get a small, temporary dipole moment like the small, temporary imbalance of nitrogen molecules inside the box.

These temporary dipoles can then, temporarily, induce dipoles in neighboring atoms so that at any given time dipole-dipole attractive forces are operating between at least some pairs of atoms. That enables condensation if you make the energy level (temperature) low enough. But it would have to be much lower temperature than if you had larger, permanent dipoles.