The Nature of Covalent Bonding

Bond Dissociation Energies A large quantity of heat is produced when atoms combine. This release of heat suggests that the product is more stable than the reactants. The total energy required to break the bond between two covalently bonded atoms is known as the bond dissociation energy.

Bond Dissociation Energies Bond dissociation energy is usually expressed in terms of the energy for every 6.02 x 1023 molecules (Avagadros # = 1 mol). A large bond dissociation energy corresponds to a strong covalent bond. EX: C-C bond has a bond dissociation energy of 347 kJ/mol.

Resonance The actual bonding of oxygen atoms in ozone is a hybrid, or mixture, of the extremes represented by the resonance forms. A resonance structure is a structure that occurs when it is possible to write two or more valid electron dot formulas that have the same number of electron pairs for a molecule or ion.

Resonance Resonance structures are simply a way to envision the bonding in certain molecules. Although no back and forth changes occur, double headed arrows are used to connect resonance structures: EX: :O:O::O ↔ O::O:O:

Exceptions to the Octet Rule The octet rule cannot be satisfied in molecules whose total number of valence electrons is an odd number. There are also molecules in which an atoms has fewer, or more, than a complete octet of valence electrons.

Exceptions to the Octet Rule The NO2 molecule contains a total of seventeen, an odd number, of valence electrons. Each oxygen contributes six electrons and the nitrogen contributes five. 2 plausible resonance structures can be written for the NO2 molecule: :O=N-O: or ∙O-N=O:

Exceptions to the Octet Rule An unpaired electron is present in each structure for NO2, both of which fail to follow the octet rule. It is impossible to write a Lewis dot structure for NO2 that satisfies the octet rule for all atoms.