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: NH4+
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 CO3 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 doubleheaded arrows between them.
Resonance Structures
-2
C
C
C