Explain MP & BP

you need to know factors affecting m.p. and b.p.

melting point is affected by both molecular mass and packing efficiency. higher molecular mass means stronger van der waal's force. more energy is required to overcome the attraction between molecules. on the other hand, packing of molecules in solid's lattice is also important. the melting point of a closely-packed solid is higher ---- as molecules gets closer, and attraction gets stronger (like inverse square law).

for chlorine, the size is small, so the packing is more efficient.
for pentane, the C-C bonds can freely rotate, so degree of freedom is high. shape of molecules is not fixed, some straight and some bent. packing of these irregular molecules cannot be efficient, so melting point is low.


boiling point depends only on dipole moment (as liquid has no lattice). since both chlorine and pentane are non-polar, dipole arises from instantaneously.
molecules with larger size (not mass) has larger electron cloud, thus magnitude of charge separation is larger.

pentane has larger electron cloud, thus stronger dipole moment, more energy to overcome the attraction, and eventually higher boiling point.