Generally, London dispersion forces depend on the atomic or molecular weight of the material. In large atoms, they can be very big, because the atoms are very soft and easy to polarize. In light atoms, they are very small, because there aren't many electrons and they are held tightly. London dispersion forces are always present, but they vary widely in strength.

Lots of induced dipoles can create attraction between molecules, called London dispersion forces. These are called induced dipoles, because they appear in response to the original accidental dipole. If they all happen to move one direction, creating a temporary dipole, the other molecules nearby can adjust, making more dipoles to attract the first one. It's electrons are far away from the nucleus, and they can move around easily. In F 2, both F atoms are holding all the electrons really tightly, trying to grab them and not share. An example of London dispersion forces for one helium atom causing a dipole to be created on a nearby helium atom.įluorine is really really hard. Elements that can't polarize easily (which usually means low atomic number) are called 'hard'. In other words, it can squish and change shape. This gives them more flexibility to move around and create temporary dipole moments.) The technical word for an element that is polarizable, or able to have temporary dipoles, is 'soft'. (Basically, the energy gaps between orbitals become smaller as we move to higher shells, allowing the electrons to more easily move into excited states, occupying orbitals higher than they need to. The more electrons an atom has, the more easily this can happen, because the electrons are held more loosely, far from the nucleus. Because the electrons move around a lot, sometimes they may move in a way that creates a temporary dipole moment. 'Dispersion' means the way things are distributed or spread out. London dispersion forces can explain how liquids and solids form in molecules with no permanent dipole moment. We mentioned this before, when we talked about polarizability. But actually, although I 2 has no permanent dipole moment, it can have a temporary dipole moment. But I 2 has no dipole moment to make attractions between the molecules. F 2 and Cl 2 are gases, Br 2 is a liquid, and I 2 is a solid at room temperature. It's not too hard to see why dipole-dipole forces hold molecules like HF or H 2O together in the solid or liquid phase.

Periodic table molar mass cl2