# 10: Bonding in Polyatomic Molecules


Valence bond (VB) theory is one of two basic bonding theories, along with molecular orbital (MO) theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds when a molecule is formed. In contrast, molecular orbital theory, which will be discussed elsewhere, predict wavefunctions that cover the entire molecule.

• 10.1: Hybrid Orbitals Account for Molecular Shape
The shape and bonding valecies of polyatomic molecules can be accounted for by hybrid orbitals. Molecular orbitals are formed from linear combinations of atomic orbitals which are similar in energy. These atomic orbitals could come from di erent atoms, or from the same atom. For example, the 2 sand 2patomic orbitals are very close energetically. When a linear combo of more than one atomic orbital from the same atom is formed, we have a hybrid orbital
• 10.2: Different Hybrid Orbitals Used for Bonding in Water
The goal of applying Valence Bond Theory to water is to describe the bonding in H2O and account for its structure (i.e., appropriate bond angle and two lone pairs predicted from VSEPR theory).  This means applying a localize two-atom bonding approach, which requires introducing hybrid orbitals to describe the experimentally observed bent structure.
• 10.3: Why is BeH₂ Linear and H₂O Bent?
Walsh correlation diagram is a plot of molecular orbital energy as a function of some systematic change in molecular geometry. For example, the correlation between orbital energies and bond angle for an $$AH_2$$ molecule. The geometry of a molecule is determined by which possible structure is lowest in energy. We can use the Walsh diagram to determine the energy trends based on which orbitals are occupied.
• 10.4: Photoelectron Spectroscopy
A photoelecton spectrum can show the relative energies of occupied molecular orbitals by ionization. (i.e. ejection of an electron). A photoelectron spectrum can also be used to determine energy spacing between vibrational levels of a given electronic state. Each orbital energy band has a structure showing ionization to different vibrational levels.
• 10.5: The $$\pi$$-Electron Approximation of Conjugation
Molecular orbital theory has been very successfully applied to large conjugated systems, especially those containing chains of carbon atoms with alternating single and double bonds. An approximation introduced by Hü​ckel in 1931 considers only the delocalized p electrons moving in a framework of $$\pi$$-bonds. This is, in fact, a more sophisticated version of a free-electron model.
• 10.6: Butadiene is Stabilized by a Delocalization Energy
Delocalization energy is intrinsic to molecular orbital theory, since it results from breaking the two-center bond concept. This is intrinsic to molecular orbital theory with the molecular orbitals spreading further than just one pair of atoms. However, within the two-center theory of valence bond theory, the delocalization energy results from a stabilization energy attributed to resonance.

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