Each of the three lines represents the shared pair of electrons in a covalent bond. When lines are used to represent bonding pairs of electrons, the structure is often called a structural formula. Some textbooks omit the nonbonding electrons for simplicity.
Figure 7. The hydrogen chloride molecule.
The
The hydrogen and chlorine atoms each donate one electron to the covalent bond. In the molecule, the hydrogen has completed its valence shell with two electrons, and the chlorine has a full shell with eight valence electrons.
In some molecules, bonded atoms share more than two electrons, as in ethylene (C 2H 4), where the two carbons share four electrons. (See Figure 8.)
Figure 8. A double bond between two carbon atoms.
Notice that each carbon achieves eight electrons by this sharing. Because each shared pair constitutes a single covalent bond, the two shared pairs are called a double bond. The structure on the right side of Figure 8 shows this double bond of four shared electrons with two lines, and the left side of Figure
shows the double bond as two pairs of dots.
There are even triple bonds of six shared electrons, as in the nitrogen molecule. In N 2, each nitrogen atom contributes five valence electrons. Of the 10 electrons shown in Figure 9, four are nonbonding, and six comprise the triple bond holding the nitrogen atoms together.
Figure 9. A triple bond between two nitrogen atoms.
- Look at the periodic table and deduce the number of valence electrons for aluminum and oxygen from the positions of the columns for those two elements.
- Draw a Lewis diagram representing the electron configuration of the hydrogen sulfide molecule, H 2S.