Multiple Covalent Bonds

Learning Objectives

  • Define double bond.
  • Define triple bond.
  • Draw Lewis electron dot structures for compounds containing double or triple bonds.

What do you do with your leftovers?

Electron symbols in a box

When working with covalent structures, it sometimes looks like you have leftover electrons. You apply the rules you learned so far and there are still some electrons hanging out there unattached. You can’t just leave them there. So where do you put them?

Multiple Covalent Bonds

Some molecules are not able to satisfy the octet rule by making only single covalent bonds between the atoms. Consider the compound ethene, which has a molecular formula of C 2 H 4 . The carbon atoms are bonded together, with each carbon also being bonded to two hydrogen atoms.

& text{two C atoms} = 2 times 4 = 8 text{valence electrons} \& text{four H atoms} = 4 times 1 = 4 text{valence electrons} \& text{total of 12 valence electrons in the molecule}

If the Lewis electron dot structure was drawn with a single bond between the carbon atoms and with the octet rule followed, it would look like this:

Incorrect Lewis dot structure of ethene

Figure 1. Incorrect dot structure of ethene.

 

This Lewis structure is incorrect because it contains a total of 14 electrons. However, the Lewis structure can be changed by eliminating the lone pairs on the carbon atoms and having the share two pairs instead of only one pair.

Correct Lewis dot structure of ethene

Figure 2. Correct dot structure for ethene.

 

A double covalent bond is a covalent bond formed by atoms that share two pairs of electrons. The double covalent bond that occurs between the two carbon atoms in ethane can also be represented by a structural formula and with a molecular model as shown below.

Structural and molecular model of ethene

Figure 3. (A) The structural model for C2H4 consists of a double covalent bond between the two carbon atoms and single bonds to the hydrogen atoms. (B) Molecular model of C2 H4.

A triple covalent bond is a covalent bond formed by atoms that share three pairs of electrons. The element nitrogen is a gas that composes the majority of Earth’s atmosphere. A nitrogen atom has five valence electrons, which can be shown as one pair and three single electrons. When combining with another nitrogen atom to form a diatomic molecule, the three single electrons on each atom combine to form three shared pairs of electrons.

A triple bond in nitrogen gas

Figure 4. Triple bond in N2.

 

Each nitrogen atom follows the octet rule with one lone pair of electrons and six electrons that are shared between the atoms.

Summary

  • Lewis structures can be drawn for molecules that share multiple pairs of electrons.

Practice

Read the material at the link below and take the quiz (Test Bite).

http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/atomic/covalentrev1.shtml

Exercises

  1. Why is the first ethene Lewis structure incorrect?
  2. What do the single electrons in nitrogen do to for a triple bond?
  3. Draw the Lewis structure for ethyne C 2 H 2 .

 Glossary

  • double covalent bond: A covalent bond formed by atoms that share two pairs of electrons.
  • triple covalent bond: A covalent bond formed by atoms that share three pairs of electrons.