3

Bonding

These are the skills that are usually tested on the SAT Subject Test in Chemistry. You should be able to . . .

• Define ionic and covalent bonds, and explain how they form.
• Identify the differences in the continuum that exists between ionic and covalent bonding.
• Describe the implications of the type of bond on the structure of the compound.
• Explain the implications of intermolecular forces and van der Waals forces.
• Explain how VSEPR and hybridization solve the need to comply with known molecular shapes.

This chapter will review and strengthen these skills. Be sure to do the Practice Exercises at the end of the chapter.

Some elements show no tendency to combine with either like atoms or other kinds of elements. These elements are said to be monatomic molecules; three examples are helium, neon, and argon. A molecule is defined as the smallest particle of an element or a compound that retains the characteristics of the original substance. Water is a triatomic molecule since two hydrogen atoms and one oxygen atom must combine to form the substance water with its characteristic properties. When atoms do combine to form molecules, there is a shifting of valence electrons, that is, the electrons in the outer energy level of each atom. Usually, this results in the completion of the outer energy level of each atom followed by their mutual attraction. This is called a chemical bond. When a chemical bond forms, energy is released; when this bond is broken, energy is absorbed.

This relationship of bonding and the valence electrons of atoms can be further explained by studying the electron structures of the atoms involved. As already mentioned, the noble gases are monatomic molecules. The reason can be seen in the electron distributions of these noble gases as shown in Table 3.1.

Table 3.1 Noble gas electron configurations

Noble Gas		Electron Distribution		Electrons in Valence Energy Level
Helium		1s2		2
Neon		1s2 2s2 2p6		8
Argon		1s2 2s2 2p6 3s2 3p6		8
Krypton		1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6		8
Xenon		1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6		8
Radon		1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6		8

The distinguishing factor in these very stable configurations, i.e., one that would take a significant amount of energy to change, is the arrangement of two s electrons and six p electrons in the valence energy level in five of the six atoms. (Note that helium, He, has only a single s valence energy level, which is filled with two electrons, making He a very stable atom.) This arrangement is called a stable octet. All other elements, those other than the noble gases, have one to seven electrons in their outer energy levels. These elements are reactive to varying degrees. When they do react to form chemical bonds, usually the electrons shift in such a way that stable octets form. In other words, in bond formation, atoms usually attain the stable electron structure of one of the noble gases. The type of bond formed is directly related to whether this structure is achieved by gaining, losing, or sharing electrons.

Tip

Notice the recurrence of the octet (8) of electrons in noble gases.