Covalent Bonds
Because nonmetals do not form monatomic cations, the nature of bonds
between atoms of nonmetals puzzled scientists until 1916, when Lewis published
his explanation. With brilliant insight, and before anyone knew about
quantum mechanics or orbitals, Lewis proposed that a covalent bond is a pair
of electrons shared between two atoms (3). The rest of this chapter and the next
develop Lewis’s vision of the covalent bond. In this chapter, we consider the
types, numbers, and properties of bonds that can be formed by sharing pairs of
electrons. In Chapter 4, we see how to understand Lewis’s concept in terms of
orbitals.
3.5 Lewis Structures
When ionic bonds form, the atoms of one element lose electrons and the atoms of
the second element gain them until both types of atoms have reached a noble-gas
configuration. The same idea can be extended to covalent bonds. However, when
a covalent bond forms, atoms share electrons until they reach a noble-gas configuration.
Lewis called this principle the octet rule:
In covalent bond formation, atoms go as far as possible toward completing
their octets by sharing electron pairs.
For example, nitrogen (åNz)
z
z has five valence electrons and needs three more electrons
to complete its octet. Chlorine (åCl z) ¥¥
has seven valence electrons and needs
one more electron to complete its octet. Argon (åAr å) ¥¥ already has a complete octet
and has no tendency to share any more electrons. Hydrogen (Hz) needs one more
electron to reach its heliumlike duplet. Because hydrogen completes its duplet by
sharing one pair of electrons, we say that it has “a valence of 1” in all its compounds.
In general, the valence of an element is the number of bonds that its
atoms can form.
We can extend the Lewis symbols introduced in Section 3.2 to describe covalent
bonding by using a line (ß) to represent a shared pair of electrons. For example,
the hydrogen molecule formed when two Hz atoms share an electron pair
(H a H) is represented by the symbol HßH. A fluorine atom has seven valence electrons
and needs one more to complete its octet. It can achieve an octet by accepting
a share in an electron supplied by another atom, such as another fluorine atom: