The states of matter and melting points of Group 17 (halogens) can be explained by the presence of intermolecular forces between molecules. The statement that best describes the reason for the increasing melting points going down the group is

 

A. iodine and bromine have weaker London forces compared to chlorine

 

B. iodine and bromine have stronger London forces compared to chlorine

 

C. iodine and bromine have dipole-dipole forces whereas chlorine does not

 

D. the number of electrons and molecular masses of the halogen family increases as you move up the group

London dispersion forces are weak attractive forces formed between atoms or molecules. These attractive forces occur when electrons in two adjacent atoms in different molecules occupy positions that makes the atom form temporary dipoles.

Larger and heavier molecules have stronger London dispersion forces. This is because as the size of the molecule increases, the valence electrons are farther and farther away from the nucleus. This means that the valence electrons are less attracted by the nucleus and are thus loosely held. As a result, these valence electrons more readily and easily form temporary dipoles.

The result of the increased attraction of the molecules to each other is that more heat energy is required to seperate them from each other.

Melting point increases down the group because as the size of the atom increases down the group, the strength of London dispersion forces increases down the group.

Iodine and Bromine have a larger atomic size than chlorine. Therefore they both have stronger London dispersion forces than Chlorine. Thus, their melting points will be greater than that of chlorine.