Question

What are induced dipole-dipole forces?

Answer 1

It's just another phrase for Van-der-Waals forces

Explanation:

These forces exist between non polar molecules or atoms. Since the molecules are non polar, the electron cloud would generally be evenly distributed between the two atoms. Think of the electron cloud as a group of electrons moving moving around in the atom on their orbitals.

The electron cloud is not stationary. It moves all about the atom very quickly. Therefore, at any given point in time, the cloud may be more concentrated on one side of the molecule than on the other. Thus the side which is more heavily concentrated with electrons will become `delta ^+` while the side with a lower concentration will become `delta ^-` (this is called a temporary dipole moment*) . These charges are only slight because they're separated by a small distance. The diagram below should provide some clarification.

Each oval represents a molecule. When the `delta ^+` end of molecule A (to the left) comes close to molecule B, the positive charge will induce a negative charge in B (keep in mind that opposite charges attract, so `delta ^+`A will force the left side of B to become negative). Thus, the electron cloud in B will be attracted to the left by molecule A, causing the right side of molecule B to become slightly positive.

This causes a weak force of attraction to form between the two molecules. The same principle applies to the `delta ^-` end of the molecules.

Temporary dipole moment - we refer to the molecule as having a temporary dipole moment because the electron cloud is constantly* moving. As the electron cloud continues to move, the direction of polarity in the atoms and molecules changes but the intermolecular forces of attraction (van-der-waals) remain.