Covalent Bonds and Molecular Structure
Interactive Student Tutorial

 7.5 Electron-Dot Structures

The electrons shared in covalent bonds can be represented using electron-dot structures (or Lewis structures) in which each dot represents a valence electron. The placement of the dots reflects the distribution of the electrons in a molecule. For example, each hydrogen atom has an electron-dot symbol H.. The combination of two hydrogen atoms to form a hydrogen-hydrogen covalent bond can be represented by the electron-dot structure H:H. By sharing two electrons in a covalent bond, each hydrogen effectively has one electron pair. This is equivalent to the filled-shell electron configuration of helium, and a filled-valence shell for each atom in a molecule leads to maximum stability.

Atoms other than hydrogen also form covalent bonds by sharing electron pairs. The electron-dot structure for the fluorine molecule is shown below.

Main group elements form covalent bonds by sharing electron pairs. Unshared or lone pairs around the central atom contribute to the octet of electrons. Table 7.3 summarizes the trends for the elements in the second row.

You may wonder why elements in group 5A do not form five bonds, since the elements in group 3A form three bonds and the elements in group 4A form four bonds. Let's consider ammonia, NH3. Nitrogen has five valence electrons,

and the ammonia molecule has one non-bonding pair and three bonds.

Thus, ammonia has an octet of electrons by forming three bonds. Elements in Group 6 and 7 similarly achieve an octet configuration by sharing pairs and lone pairs.


 Electron-Dot Structures I



More than one pair of electrons can be shared in a covalent molecule, leading to the formation of multiple covalent bonds. When two pairs of electrons are shared, the result is a double bond. Similarly, the sharing of three pairs results in a triple bond. The term bond order is used to refer to the number of shared electron pairs. A double bond has a bond order of 2 and a triple bond has a bond order of 3. Multiple bonds are both shorter and stronger than their corresponding single bonds because there are more shared electrons holding the atoms together.

A different kind of covalent bond is formed when one atom contributes both electrons in a shared pair. The atoms nitrogen, oxygen, phosphorus, and sulfur frequently form these coordinate covalent bonds, which we will consider in more detail in the Interactive Student Tutorial section of Chapter 20.5.