Covalent Bonding An Introduction

To emphasize the sharing of electrons, let us think of the Lewis structure of HCl in this manner. 

The broken circles represent the outermost electron shells of the bonded atoms. The number of dots lying on or within each circle represents the effective number of electrons in each valence shell. The H atom has two dots, as in the electron configuration of He. The Cl atom has eight dots, corresponding to the outer-shell configuration of Ar. Note that we counted the two electrons between H and Cl ) twice. These two electrons are shared by the H and Cl atoms. This shared pair of electrons constitutes the covalent bond. Written below are two additional Lewis structures of simple molecules. 

As was the case for Cl in HCl, the O atom in the Lewis structure of H2O and in CI2O is surrounded by eight electrons (when the bond-pair electrons are double counted). In attaining these eight electrons, the O atom conforms to the octet rule—a requirement of eight valence-shell electrons for the atoms in a Lewis structure. Note, however, that the H atom is an exception to this rule. The H atom can accommodate only two valence-shell electrons. 

Lewis theory helps us to understand why elemental hydrogen and chlorine exist as diatomic molecules, H2 and CI2. In each case, a pair of electrons is shared between the two atoms. The sharing of a single pair of electrons between bonded atoms produces a single covalent bond. To underscore the importance of electron pairs in the Lewis theory the term bond pair applies to a pair of electrons in a covalent bond, while lone pair applies to electron pairs that are not involved in bonding. Also, in writing Lewis structures it is customary to replace bond pairs with lines (—). These features are shown in the following Lewis structures. 

The Lewis theory of bonding describes a covalent bond as the sharing of a pair of electrons, but this does not necessarily mean that each atom contributes an electron to the bond. A covalent bond in which a single atom contributes both of the electrons to a shared pair is called a coordinate covalent bond. 

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