Homonuclear Diatomic Molecules of the Period 2 Elements

Homonuclear diatomic molecules are those composed of two identical atoms. In addition to H2 from Period 1, you re also familiar with several from Period 2—N2, O2, and F2—as the elemental forms under standard conditions. Others in Period 2—U2, Be2, B2, C2, and Ne2—are observed, if at all, only in high-temperature gas-phase experiments. Molecular orbital descriptions of these species provide some interesting tests of the model. Let s look first at the molecules from the block. Groups 1 A( 1) and 2A(2), and then at those from the p block. Groups 3A(13) through 8A(18). 

Bonding in the s-Block Homonuclear Diatomic Molecules Both Li and Be occur as metals under normal conditions, but let s see what MO theory predicts for their stability as the diatomic gases dilithium (Li2) and diberyllium (Be2). 

The order of MO energy levels, whether bonding or antibonding, is based on the order of AO energy levels ncl on the mode of the p-orbital combination  

the energy order for MOs derived from 2p orbitals is 

The B2 molecule has six outer electrons to place in its MOs. Four of these fill the U2s and MOs. The remaining two electrons occupy the two TT2p MOs, one in each orbital, in keeping with Hund s rule. With four electrons in bonding MOs and two electrons in antibonding MOs, the bond order of B2 is j(4 - 2) = I. As expected from its MO diagram, B2 is paramagnetic. 

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Molecular Orbital (MO) Theory and Electron Delocalization Central Themes of MO Theory Homonuclear Diatomic Molecules of the Period 2 Elements... 

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