Properties and Covalent Bonding of Molecular Substances

A practical consequence of these properties is that many molecular elements and compounds do not dissolve readily in water.

The Nature of Molecular Compounds: Covalent Bonding

To understand why molecular substances behave this way, we must look at the microscopic level to see how they are held together. The fundamental "glue" of a molecular substance is the covalent bond.

Covalent Bonding and the Shared Electron Pair

Covalent bonding occurs when atoms interact to achieve stability by sharing electrons. This process can be visualized as a "tug-of-war" between atoms.

Consider the formation of a hydrogen molecule ($H_2$). A single hydrogen atom consists of one proton and one electron. When two hydrogen atoms collide, the positive charge of the proton in one atom exerts an attractive force on the negative electron of the other atom, and vice versa. Instead of one atom "stealing" the electron, they settle into a stable arrangement where they both share a shared electron pair. This shared pair acts as the attractive force that holds the two atoms together.

Figure 3: Representation of a covalent bond between two hydrogen atoms.

This principle applies to molecular elements as well. For instance, chlorine is a diatomic element, meaning it exists as $Cl_2$ molecules. In a chlorine molecule, the two chlorine atoms bond in a way that allows each atom to achieve the stable electron arrangement typically found in a noble gas.

LEARNING TIP: The prefix co- in "covalent" typically means "with" or "shared" (as in cooperate or cohabit). Therefore, "covalent" literally implies "sharing its valence" or "sharing electrons".

Bonding Capacity and Lewis Symbols

Not all atoms share the same number of electrons. The ability of an atom to form bonds is determined by its bonding electrons—those located in the valence shell that are available to participate in a covalent bond.

Bonding Capacity

The number of bonds an atom can form is known as its bonding capacity. This capacity is directly related to the number of valence electrons the atom possesses.

Table 2: Bonding capacities and Lewis symbols of common atoms.

Visualizing Bonds with Lewis Symbols

Chemists use Lewis symbols to represent the valence electrons of an atom. To represent the actual bonds within a molecule, a Lewis structure is used. In these structures:

• A single dash (—) represents a pair of shared bonding electrons.
• Dots are used to represent unshared electrons (lone pairs).

By using these symbols, scientists can create a two-dimensional model of how atoms are connected and how many electrons are being shared between them.