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: NH 4 +
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 NH 4+/NH 3 Ammonium/Ammonia Cr. O 4 -2 Chromate C 2 H 3 O 2 - Acetate Cr 2 O 7 -2 Dichromate CH 3 COO- Acetate Mn. O 4 - Permanganate CN- Cyanide Mn. O 4 -2 Manganate CO 3 -2 Carbonate NO 2 - Nitrite HCO 3 - Bicarbonate NO 3 - Nitrate C 2 O 4 -2 Oxalate OH- Hydroxide Cl. O- Hypochlorite PO 4 -3 Phosphate Cl. O 2 - Chlorite SCN- Thiocyanate Cl. O 3 - Chlorate SO 3 -2 Sulfite Cl. O 4 - Perchlorate SO 4 -2 Sulfate S 2 O 3 -2 Thiosulfate HSO 4 - Hydrogen sulfate Br. O- Hypobromite IO 3 - Iodate As. O 2 -3 Arsenite Se. O 4 -2 Selenate Br. O 3 - Bromate HSO 3 - Hydrogen sulfite
Try it: CO 3 -2 C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e- O O C O
Try it: CO 3 -2 C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e- O O C O
Try it: CO 3 -2 C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e- O O C O
Try it: CO 3 -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: CO 3 -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 CO 3 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. O 3 NO 3 -1 HCO 2 -1 CN-1 SO 32 - 6. Br. O-1 7. PO 438. IO 3 -1 9. CHO 2 -1 10. NO 2+1
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