London (dispersion) force

[Monica Soliman 3F]

I understand why C2H6 and I2 are dispersion forces, however, I m not quite sure why BCl3 is? Isn't it polar bonding between B and Cl?

[Jonathan Batac - 2D]

Although we haven't learned this yet, BCl3 has a symmetric shape (trigonal planar). You can kind of think of each Chlorine equally pulling Boron in separate directions, so therefore there is no polar movement of the molecule. You can also apply this thinking to I2; the Iodines are equally repelling each other so there is no polar movement. Since there is no polar movement, BCl3 doesn't exhibit any dipole movement, and this leaves LDF as the only option.