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Molecules respond to the many wavelengths of light. The wavelengths that are

Molecular Orbitals. -- The VSEPR and valence-bond theories don’t explain the excited states of molecules, which come into play when molecules absorb and emit light. -- This is one thing that the molecular orbital (MO) theory attempts to explain. . Molecules respond to the

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Molecules respond to the many wavelengths of light. The wavelengths that are

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  1. Molecular Orbitals -- The VSEPR and valence-bond theories don’t explain the excited states of molecules, which come into play when molecules absorb and emit light. -- This is one thing that the molecular orbital (MO) theory attempts to explain. Molecules respond to the many wavelengths of light. The wavelengths that are absorbed and then re-emitted determine an object’s color, while the wavelengths that are NOT re-emitted raise the temperature of the object.

  2. molecular orbitals: wave functions that describe the locations of electrons in molecules -- these are analogous to atomic orbitals in atoms (e.g., 2s, 2p, 3s, 3d, etc.), but MOs are possible locations of electrons in molecules (not atoms) -- MOs, like atomic orbitals, can hold a maximum of two e– with opposite spins -- but MOs are for entire molecules MO theory is more powerful than valence-bond theory; its main drawback is that it isn’t easy to visualize.

  3. (antibonding MO) s*1s 1s 1s The overlap of two atomic orbitals produces two MOs. Hydrogen (H2) s1s + (bonding MO) H2 molecular orbitals H atomic orbitals -- The lower-energy bonding molecular orbital concentrates e– density between nuclei. -- For the higher-energy antibonding molecular orbital, the e– density is concentrated outside the nuclei. -- Both of these are s molecular orbitals.

  4. (antibonding MO) s*1s 1s 1s s1s + (bonding MO) H2 molecular orbitals H atomic orbitals Energy-level diagram (molecular orbital diagram) s*1s 1s 1s s1s H atom H atom H2 m’cule

  5. Consider the energy-level diagram for the hypothetical He2 molecule… s*1s 1s 1s s1s He atom He atom He2 m’cule 2 bonding e–, 2 antibonding e– No energy benefit to bonding. He2 molecule won’t form.

  6. 1 bonding e–, zero antibonding e– bond order = ½ (# of bonding e– – # of antibonding e–) -- the higher the bond order, the greater the bond stability no bond -- a bond order of... 0 = 1 = single bond 2 = double bond 3 = triple bond 007 = 7 = James Bond -- MO theory allows for fractional bond orders as well. What is the bond order of H2+? 1 e– total BO = ½ (1–0) = ½

  7. 3 4 5 6 7 8 9 10 Li Be B C N O F Ne 6.941 9.012 10.811 12.011 14.007 15.999 18.998 20.180 Second-Row Diatomic Molecules 1. # of MOs = # of combined atomic orbitals 2. Atomic orbitals combine most effectively with other atomic orbitals of similar energy. 3. As atomic orbital overlap increases, bonding MO is lowered in energy, and the antibonding MO is raised in energy. 4. Both the Pauli exclusion principle and Hund’s rule apply to MOs.

  8. s*2s 2s 2s s2s s*1s 1s 1s s1s Use MO theory to predict whether Li2 and/or Be2 could possibly form. Li Li Li2 BO = ½ (4–2) = 1 “YEP.”

  9. s*2s 2s 2s s2s Bonding and antibonding e– cancel each other out in core energy levels, so any bonding is due to e– in bonding orbitals of outermost shell. Be Be Be2 BO = ½ (4–4) = 0 “NOPE.”

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