London dispersion forces ...
London dispersion forces (LDF) exist between all molecules. The strength of LDF's is proportional to the polarizability of the molecule, which in turn, depends on the number of electrons and the surface area of the molecule. Sometimes London dispersion forces are characterized as the weakest of the three van der Waals forces. In actuality, London dispersion forces are often stronger than Keesom or Debye forces and are second only to hydrogen bonding. The strength of London dispersion forces does not depend on the molar mass. That is only coincidental since "heavier" molecules often contain more electrons and are larger.
1) CH3CH2CH2CH2CH3... C5H12
2) CH3C(CH3)2CH3... C5H12
3) CH3CH(CH3)C(CH3)2CH2CH3... C8H18
4) CH3CH2(CH2)4CH2CH3... C8H18
5) CH3CH2CH3... C3H86) CH4
--- Increasing London dispersion forces --->
CH4 ... CH3CH2CH3 ... CH3C(CH3)2CH3 ... CH3CH2CH2CH2CH3 ... CH3CH2(CH2)4CH2CH3 ... CH3CH(CH3)C(CH3)2CH2CH3
The C8H18 molecules have the strongest London forces. The less compact molecules have greater surface area and greater LDFs.
Other intermolecular forces
Keesom forces (dipole-dipole attraction) ... the attraction of one polar molecule for another. Oppositely charged ends of the molecules undergo electrostatic attraction.
Debye forces (induced attraction) .... the attraction between a polar molecule and one which is nonpolar, where the polar molecule induces a charge separation in the nonpolar molecule. Debye forces can also exist between two polar molecules, even the same kind of molecule. In such a case the induced charge may momentarily increase or decrease the dipole moment of the substance.
Hydrogen bonding .... The weakly covalent bonds found between the hydrogen atom of one molecule where it is bonded to N, O or F, and the N, O, or F of an adjacent molecule. The hydrogen atom functions as a bridge, forming a bond between two molecules. Hydrogen bonding is more than an electrostatic attraction between molecules, instead there is evidence of orbital overlap and covalent bond formation.
Calculate the Mw of each molecules, the higher it is, the more stronger the london forces it will be between the molecules.