KEY2CHEM

Structure and Properties of Molecular Solids

Molecular solids are composed of nonmetal elements or compounds that experience weak intermolecular forces between the particles. Because the particles are held together by relatively weak interactions, molecular solids generally have low melting points. Molecular solids are also generally not good conductors, since their electrons are localized within covalent bonds.

Example 1.

Which is NOT a property of molecular solids?

A. low melting point

B. not a good conductor

C. brittle

Solution

C. brittle

Molecular solids generally have low melting points and are non-conducting, due to their covalent bonds. The weak intermolecular forces holding particles together within a molecular solid generally allow the particles to slide past one another, unlike brittle ionic solids.

Example 2.

Which solid is a molecular solid?

A. sucrose ($$\require{mhchem}\ce{C12H22O11}$$)

B. sodium chloride ($$\require{mhchem}\ce{NaCl}$$)

C. copper ($$\require{mhchem}\ce{Cu}$$)

Solution

A. sucrose ($$\require{mhchem}\ce{C12H22O11}$$)

Sucrose is a covalent compound that forms a molecular solid.

Example 3.

Which solid is expected to have the lowest melting point?

A. silver ($$\require{mhchem}\ce{Ag}$$)

B. potassium chloride ($$\require{mhchem}\ce{KCl}$$)

C. glucose ($$\require{mhchem}\ce{C6H12O6}$$)

Solution

C. glucose ($$\require{mhchem}\ce{C6H12O6}$$)

Glucose is a molecular solid and is expected to have a lower melting point than a metallic solid or ionic solid.