Of the compounds shown below, the one with the lowest melting point would likely be : A. CF4 B. CH4 C. CI4 (thats an i not an L) D. CBr4 E. CCl4 Why? Solution All of them have one carbon only. The difference is in the sizes of halides and H along with number of electrons. As size and molecular weight increases greater energy is required to give molecules energy for increasing their kinetic energy and internal energy because liquid molecules have greater mobility than solid. CH4 has lowest molecular weight and size. Also as atomic size increases with increasing atomic number the polarizability increases and thus attractive london dispersion forces increase. H being least in atomic size and atomic number, CH4 will have least london dispersion forces of attraction and less forces to hold CH4 molecules together. Hence CH4 will have least melting point. Comment if any doubt. .