19.11: Physical Properties and Intermolecular Forces

Figure $\PageIndex{1}$ (Public Domain; Moskey71 via https://commons.wikimedia.org/wiki/F...opeDiamond.JPG)

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Carbon is an interesting and versatile element. There are over twenty million known compounds containing carbon, encompassed in the growing field of organic chemistry. The element itself can exist in two major forms. Diamond is a form of carbon that is extremely hard and is one of the few materials that can scratch glass. The other form of carbon is graphite, a very soft material that we find in “lead” pencils. The two forms differ mainly in how the carbon atoms are connected to one another. The differences in the arrangement of atoms affect the properties of the material.

Physical Properties and Intermolecular Forces

The physical state and properties of a particular compound depend in large part on the type of chemical bonding it displays. Molecular compounds, sometimes called covalent compounds, display a wide range of physical properties due to the different types of intermolecular attractions such as different kinds of polar interactions. The melting and boiling points of molecular compounds are generally quite low compared to those of ionic compounds. This is because the energy required to disrupt the intermolecular forces between molecules is far less than the energy required to break the ionic bonds in a crystalline ionic compound. Since molecular compounds are composed of neutral molecules, their electrical conductivity is generally quite poor, whether in the solid or liquid state. Ionic compounds do not conduct electricity in the solid state because of their rigid structure, but conduct well when either molten or dissolved into a solution. The water solubility of molecular compounds is variable and depends primarily on the type of intermolecular forces involved. Substances that exhibit hydrogen bonding or dipole-dipole forces are generally water soluble, whereas those that exhibit only London dispersion forces are generally insoluble. Most, but not all, ionic compounds are quite soluble in water. The table below summarizes some of the differences between ionic and molecular compounds.

Table $\PageIndex{1}$: Comparison of Ionic and Molecular Compounds

Property	Ionic Compounds	Molecular Compounds
Type of elements	metal and nonmetal	nonmetals only
Bonding	ionic – transfer of electron(s) between atoms	covalent – sharing of pair(s) of electrons between atoms
Representative unit	formula unit	molecule
Physical state at room temp.	solid	gas, liquid, or solid
Water solubility	usually high	variable
Melting and boiling temps	generally high	generally low
Electrical conductivity	good when molten or in solution	poor

One type of molecular compound behaves quite differently than that described so far. A covalent network solid is a compound in which all of the atoms are connected to one another by covalent bonds. Diamond is composed entirely of carbon atoms, each bonded to four other carbon atoms in a tetrahedral geometry. Melting a covalent network solid is not accomplished by overcoming the relatively weak intermolecular forces. Rather, all of the covalent bonds must be broken, a process that requires extremely high temperatures. Diamond, in fact, does not melt at all. Instead, it vaporizes to a gas at temperatures above 3500°C.

Summary

• The physical properties of a material are affected by the intermolecular forces holding the molecules together.

Review

1. Are melting points of molecular compounds generally higher or lower than those of ionic compounds?
2. Do ionic compounds conduct electricity in the solid state?
3. What types of substances are generally water-soluble?