Co-ordinate Covalent Bonds
and
Resonance Structures
Co-ordinate Covalent Bond
• A covalent bond is formed by two atoms sharing a
pair of electrons. The atoms are held together
because the 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.
Example: NH4
+
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 the same
as always
3. Put [square] brackets around the
structure
4. Write the charge as a superscript
Do you remember polyatomic ions?
Formula
Name
Formula
Name
NH4+/NH3
Ammonium/Ammonia
CrO4-2
Chromate
C2H3O2-
Acetate
Cr2O7-2
Dichromate
CH3COO-
Acetate
MnO4-
Permanganate
CN-
Cyanide
MnO4-2
Manganate
CO3-2
Carbonate
NO2-
Nitrite
HCO3-
Bicarbonate
NO3-
Nitrate
C2O4-2
Oxalate
OH-
Hydroxide
ClO-
Hypochlorite
PO4-3
Phosphate
ClO2-
Chlorite
SCN-
Thiocyanate
ClO3-
Chlorate
SO3-2
Sulfite
ClO4-
Perchlorate
SO4-2
Sulfate
S2O3-2
Thiosulfate
HSO4-
Hydrogen sulfate
BrO-
Hypobromite
IO3-
Iodate
AsO2-3
Arsenite
SeO4-2
Selenate
BrO3-
Bromate
HSO3-
Hydrogen sulfite
Try it: CO3-2
C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e-
O
O
C
O
Try it: CO3-2
C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e-
O
O
C
O
Try it: CO3-2
C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e-
O
O
C
O
Try it: CO3-2
All 24 e- have been used, buy C does NOT have a
complete octet….. Need to make a double bond….Does it
matter which O it makes the double bond with?
O
O
C
O
Try it: CO3-2
The bond can go here but it could also go
between the C and either of the other Os
O
O
C
O
Finished Product 
-2
C
C
C
Resonance Structures
• The Lewis structure for CO3 shows two different types of
bonds, single and double.
• Double bonds also have a shorter bond length, the distance
between the nuclei of the two atoms in the bond, than
single bonds do. Thus, if the above Lewis structure for
nitrate 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 (2)
• So the carbonate ion would actually 3 bonds, with the bond
lengths between the characteristic single and double bond
lengths
• The 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
Drawing them:
• Expressing resonance when drawing Lewis
structures is done by drawing each of the
possible resonance forms and placing
double-headed arrows between them
Carbonate ion:
-2
C
C
C
Try these:
1.
2.
3.
4.
5.
O3
NO3-1
HCO2-1
CN-1
SO32-
6. BrO-1
7. PO438. IO3-1
9. CHO2 -1
10. NO2+1