3.7: Multiple Bonds
- To draw electron dot structures.
- To recognize molecules that are likely to have multiple covalent bonds.
Electron Dot Structures of Molecules
Electron dot structures also called Lewis structures are drawn by pairing up the unpaired electrons on the constituent atoms so that each atom has an octet or duet complete. See these examples:
MULTIPLE BONDS
In many molecules, the octet rule would not be satisfied if each pair of bonded atoms shares only two electrons. In case of carbon dioxide, more than one pair of electrons must be shared between two atoms for both atoms to have an octet.
In this arrangement, the carbon atom shares four electrons (two pairs) with the oxygen atom on the left and four electrons with the oxygen atom on the right. There are now eight electrons around each atom. Two pairs of electrons shared between two atoms make a double bond between the atoms, which is represented by a double dash:
Triple bonds are covalent bonds in which three pairs of electrons are shared by two atoms. A compound that has a triple bond is acetylene (C 2 H 2 ), whose Lewis diagram is as follows:
The Lewis diagram for formaldehyde \(\ce{CH2O}\) is shown below. The carbon atom is the central atom. One application of formaldehyde, is the preservation of biological specimens. Aqueous solutions of CH 2 O are called formalin and have a sharp, characteristic (pungent) odor.
How many double bonds, single bonds, and lone pairs of electrons are present in the molecule of formaldehyde? Are the octet/duet rules met for each atom?
Solution
There is one double bond between C and O atom, two single bonds between C and H atoms, and two lone pairs on the oxygen atom. The octet for the oxygen and carbon are complete. The two hydrogen atoms complete their duet.
Draw the Lewis diagram for each molecule.
- \(\ce{O2}\)
- \(\ce{C2H4}\)
- Answer a:
-
or
- Answer b:
- or or
Which is the correct Lewis structure for N 2 H 2 ?
A.
B.
C.
Solution
Lewis structure A is the correct answer. Each of the N atoms satisfy the octet requirement and the H atoms follow the duet rule.
Structure B is electron deficient. It has only 10 valence electrons instead of 12.
Structure C has 14 (2 extra) electrons. The N atoms do not satisfy the octet.
Exercise \(\PageIndex{2}\)
Which is the correct Lewis structure for NOCl?
A.
B.
C.
- Answer
-
Structure A violates the octet rule; N is surrounded by only 6e - .
Structure B violates the octet rule; Cl has 10e - around it.
Structure C is the correct structure. It has a total of 6e - + 5e - + 7e - = 18e - . Each atom is surrounded by 8 electrons (octet rule).
Key Takeaways
- A Lewis structure shows the bonding and nonbonding electrons around individual atoms in a molecule.
- Some molecules must have multiple covalent bonds between atoms to satisfy the octet rule.
- A double bond contains four electrons and a triple bond contains six electrons.