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Homonuclear Diatomic Molecules
From ChemPRIME
| The molecular-orbital model can easily be extended to other diatomic molecules in which both atoms are identical (homonuclear diatomic molecules). Three general rules are followed. First, both the core orbitals and the valence orbitals of the atoms need be considered. Second, only atomic orbitals whose energies are similar can combine to form molecular orbitals. Third, the number of molecular orbitals obtained is always the same as the number of atomic orbitals from which they were derived. Let's practice writing the molecular orbital energy diagrams, electron configuration (something we haven't done previously), bond order, and stability for the diatomic molecules of Hydrogen, Helium (hypothetical), Boron, Nitrogen, and Oxygen. Hydrogen Because the only two electrons in an H2 molecule are in a bonding orbital, the bond order is one. The molecule would be stable because the overall energy of the system is quite low, and thus, quite stable. Drawing MO energy diagram of H2 Helium Two electrons of the He2 will be in the bonding orbital making the molecule more stable than the seperate atoms. But the two electrons in the antibonding orbital would make the molecule less stable than the separate atoms. These effects cancel so the molecule would be no more stable than the separate atoms. The bond order is zero, and the molecule would not exist (He2is unknown). Drawing MO energy diagram of He2 Boron This molecule illustrates the use of Hund's Rule in that the ?2py and ?2pz orbitals are equal in energy and contain a total of two electrons. The bond order is one. Drawing MO energy diagram of B2 Nitrogen Each nitrogen atom has 7 electrons, so the diamagnetic N2 molecule has 14 electrons. Six more electrons occur in bonding orbitals than in antibonding orbitals, so the bond order is three. The greater bond order indicates that this molecule has a very short bond length, only 1.09 angstroms, the shortest of any diatomic species except H2. Drawing MO energy diagram of N2 Oxygen Because there are four more electrons in bonding orbitals than in antibonding orbitals, the bond order is two. We can thus, see why the molecule is much more stable than two free O atoms. Drawing MO energy diagram of O2 Fluorine The bond order is one and as we know F2 indeed exists. The F-F bond distance is longer than the bond distances in O2 molecules. The bond order and bond energies are lower than those of the oxygen or nitrogen diatomic molecules. Thus, F2 molecules are the most reactive of the three. Drawing MO energy diagram of F2 |
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Subpages (6): Drawing the MO of F2 MO energy diagram of B2 MO energy diagram of H2 MO energy diagram of He2MO energy diagram of N2 MO energy diagram of O2
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