More Covalent Bonding Co-ordinate Covalent Bonds and Resonance Structures Chemistry 11 Ms. McGrath

Co-ordinate Covalent Bonds A covalent bond is formed by two atoms sharing a pair of electrons. The atoms are held together because the bonding electron pair is attracted by both of the nuclei. In a simple covalent bond, each atom supplies one electron to the bond - but that doesn't have to be the case. A co-ordinate bond is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.

Co-ordinate Covalent Bonds Example: NH 4 +

Co-ordinate Covalent Bonds Rules for molecules with an overall charge 1.When figuring out the number of electrons available (ie. counting the valence electrons), add or subtract electrons as indicated by the charge. 2.Create the Lewis structure following the Step 4 rules. 3.Put [square] brackets around the structure. 4. Write the charge as a superscript

Resonance Structures The Lewis structure for CO 3 shows two different types of bonds, single and double. As we know, double bonds also have a shorter bond length than single bonds do. Thus, if the above Lewis structure for carbonate were correct, the carbonate ion would have one bond that is shorter and stronger than the other two. This is not the case. All three of the bonds in the carbonate ion are the same strength and the same length. Interestingly, the behaviour of the bonds suggests they are longer than double bonds and shorter than single bonds. They are also stronger than single bonds but not as strong as double bonds.

Resonance Structures So the carbonate ion has 3 bonds, with the bond lengths between the characteristic single and double bond lengths Important points to remember about resonance forms are:  The molecule is not rapidly oscillating between different discrete forms  There is only one form of the each molecule, we need to draw two (or more) Lewis structures because a single structure is insufficient to describe the real structure

Resonance Structures Expressing resonance when drawing Lewis structures is done by drawing each of the possible resonance forms and placing double- headed arrows between them.

Resonance Structures CCC -2