Intermolecular Forces

Intramolecular forces = forces within a molecule (chemical bonds)

Intermolecular forces = forces between molecules (solids & liquids). Always weaker than intramolecular forces.

Recall the 3 types of compounds:

ionic: compound made of ions (usually metal + nonmetal), which have charges with integer values (±1 or more)

covalent: compound made by sharing of electrons (usually all nonmetals),

metallic: compound made of metal atoms with delocalized electrons

Types of intermolecular forces (IMF), strongest to weakest:

The stronger the IMFs, the higher the melting and boiling point of the compound, because you have to overcome the IMF in order to separate the molecules going from solid à liquid or liquid à gas.

ion-ion: force of attraction between ions. Strength of force is based on Coulomb’s Law:

Bigger charges (larger values ofQ) mean stronger forces. If charges are the same, smaller molecules (smaller value of d) have stronger forces.

metallic bonds: metal atoms that delocalize their electrons and are held together by the “sea” of electrons surrounding them.

dipole-dipole: the force of attraction between two polar molecules (dipoles). Strength of attraction is based on the dipole moment (μ) of the molecule:

μ = Qd

Bigger partial charges (larger Q) and larger separation of δ+ and δ− charges within the molecule (larger d) means bigger dipole moment = stronger forces.

dispersion forces: random movement of electrons causes temporary dipoles to form within molecules, causing very weak attraction. All molecules have dispersion forces, but they can only be seen in the absence of stronger forces. Because dispersion forces are attractions between electrons, larger atoms/molecules (i.e., ones with more electrons) have stronger dispersion forces.

You can also have IMFs between different types of molecules, such as ion-dipole attraction, or dipole-induced dipole attraction (i.e., attraction between a dipole and a molecule that only experiences dispersion forces).