Question

Intermolecular interactions are important in the formation of crystals, since they control the orientation of adjacent molecules. Which of the following is not considered an intermolecular force for directing crystal growth?

	1.	hydrogen bonding
	2.	van der Waals forces
	3.	molecular dipole moments
	4.	atomic polarizability

Long-range organization of molecules in the organic solid state generally results in compounds that

	1.	are generally crystalline
	2.	are generally amorphous
	3.	give broad melting points
	4.	are electronically neutral

Answer 1

1. Hydrogen bonding is an intermolecular force that comes into play when there is a hydrogen directly bonded to an electronegative atom like O, N or F. For example, in water H is directly bonded to O, this makes the -O-H bond very polar and hence the O of one water molecule forms a hydrogen bond with H of another water molecule.

Vander waal forces of attraction are intermolecular forces of attraction present between molecules of a compound or atoms of an element due to instantaneous distortions of electron cloud and resulting induced dipoles in the molecules.

Molecular dipole moments are forces of attraction that exist between molecules which have a permanent dipole moment i.e. they are polar.

Atomic polarizability is a property of an atom by which its electron cloud can be distorted. The larger the size of the atom, the more electrons it has, the more its electron cloud can be distorted, the more polarizable it is. So atomic polarizability is not an intermolecular force for directing crystal growth. So correct answer is option (4)

2. A crystalline substance has a long range order or organisation of molecules/atoms. So long range organization of molecules in the organic solid state generally results in compounds that are generally crystalline. (Option (1)).

Amorphous substances have short range order or organisation of atoms/molecules.

Crystalline solids (having long range order) usually have sharp melting points, so these don't give broad melting points.