London dispersion

Meaning: London dispersion forces come from momentary, uneven electron distribution. Electrons move constantly, so at any instant an atom or molecule has a temporary dipole. That dipole distorts the electron cloud of a neighbor (an induced dipole), and the two attract. Every atom and molecule has these forces, including nonpolar ones such as noble gases, N₂ and hydrocarbons. Strength rises with polarizability, meaning how easily the electron cloud is distorted. Larger molecules with more electrons are more polarizable, and so are molecules with more surface area available for contact.

On the MCAT: Expect boiling-point or melting-point rankings. Boiling points rise down the noble gases, and F₂ is a gas, Br₂ a liquid and I₂ a solid at room conditions. Longer alkanes boil higher than shorter ones. A straight-chain isomer boils higher than a branched isomer because it touches neighbors over more surface. Dispersion forces also explain why real gases deviate from ideal behavior at low temperature and high pressure.

Common confusion: Dispersion forces are not always weaker than dipole-dipole forces. For molecules of similar size, hydrogen bonding is stronger than dipole-dipole, which is stronger than dispersion. A much larger molecule can have greater total dispersion than a small polar one. They are also intermolecular, so boiling breaks them, not covalent bonds.