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Chapter 2.4

Chem 1100 Chapter 2.4 Review Notes.docx

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Western University
Chemistry 1027A/B
Felix Lee

2.4: Chemistry Review Major Deficiencies in VB: 1. The assumption that electrons are localized is not entirely correct. For example, in resonance hybrids the electrons are delocalized. 2. It does not provide obvious insight on the relative energies of the electrons. 3. It fails to explain certain phenomena, such as whether molecules are paramagnetic or diamagnetic. Molecular Orbital (MO) Theory – takes a delocalized approach to bonding and focuses on the wave behaviour of electrons In MO, molecular orbitals are formed by combining wave functions of atomic orbitals. This can occur in two ways: constructively and destructively Constructive Combination – occurs when two atomic orbitals in the same phase (either both + or both -) combine and form a bonding MO, but it doesn’t generate any new nodes between the nuclei. The combination increases the electron density between the two nuclei, consistent with the formation of a bond. The resulting MO is termed bonding and designated σ . 1s Destructive Combination – occurs when two atomic orbitals in opposite phases combine and it generates a new node between the two nuclei. Since there are no electrons shielding the positively charged nuclei from each other, an unfavourable Coloumbic repulsion occurs. This does not lead to bond formation and the resulting MO is termed antibonding and designated σ* . 1s Note: the 1s subscript indicates the atomic orbitals used to form the MO. Each MO holds a maximum of two electrons. MO Theory: 1. The number of MOs formed is always equal to the total number of atomic orbitals combined. 2. Bonding MOs are always lower in energy than atomic orbitals from which they originate. Antibonding MOs are always higher in energy than the originating atomic orbitals. As a result the bonding MOs stabilize the molecule (contribute to bond formation), while antibonding MOs destabilize the molecule (do not contribute to bond formation). Therefore, for any molecule to be stable (exist) it must have more bonding that antibonding electrons. 3. MOs are filled lowest energy to highest energy. The highest energy MO with one or more electrons is called the Highest Occupied Molecule Orbital (HOMO). The lowest energy MO with no electrons present is called the Lowest Unoccupied Molecule Orbital (LUMO). 4. MOs are formed most effectively when the originating atomic orbitals are identical or close in energy. Bond Order – a parameter used to describe the relative numbers of bonding to antibonding electrons in a molecule Bond Order = (number of bonding electrons) – (number of antibonding electrons)/2 Species with higher bond orders are more stable. Paramagnetic – molecules that are attracted to a magnetic field and must contain at least one unpaired electron and a nonzero net spin (sum of all m vslues cannot equal 0) Diamagnetic – molecules that are not
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