Question

How can Van der Waals forces be either attractive or repulsive?

Answer 1

Electrons are distributed throughout an atom or molecule, and these electrons are always moving. At any moment there can be uneven distributions of electrons which form a temporary dipole. This dipole causes another temporary dipole to form an atom. Molecules become attracted to each other which is referred to as Van der Waals force.

Complete answer:
- Van der Waals forces are comparatively weaker electrostatic forces that attract uncharged or neutral molecules towards each other in almost all organic liquids, gases, and solids.
- In solids, these forces hold each other having lower melting points, and are relatively softer than the ones held by ionic or metallic bonds. In comparison to most of the ionic and covalent bonds, these interactions do not arise from an electronic bond.
- The Van der Waals forces include attractions within various atoms, resulting from influenced dipoles. However, they also involve a repulsion interaction within molecules, arising from the overlapping of more than two atomic electronic clouds situated closer to each other. Furthermore, they are known as a universal interaction between various particles, divided by mediums of air or vacuum.
- Now the sum of the attractive forces with respect to the repulsive forces decides whether the force will be repulsive or attractive.
- The molecule tends to attract in case if the sum of repulsive forces is exceeded by the sum of attractive forces.
- The molecule tends to repel in case if the sum of attractive forces is exceeded by the sum of repulsive forces.

Note:
The intermolecular forces are usually attractive under normal pressure conditions. Although, molecules are closely packed to each other at high pressures. This causes the atom nuclei to repel. This repulsive force further exceeds the attractive force.