Tham khảo sách 'chemistry part 15, julia burdge,2e (2009)', khoa học tự nhiên, hoá học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 344 CHAPTER 9 Chemical Bonding II Molecular Geometry and Bonding Theories Figure Bond order determination for Li2 and Be2. 2s Li atom 2s Li2 molecule Be2 molecule Bond order 2s Be atom Bond order molecule is. The hisher the bond order the more stable the molecule. Bond order is calculated in the following way number of electrons in bonding molecular orbitals Equation bond order When we draw molecular orbital diagrams we need only show the valence orbitals and electrons. number of electrons in antibonding __molecular orbitals 2 In the case of H2 where both electrons reside in the gt1s. orbital the bond order is 2 0 2 1. In the case of He2 where the two additional electrons reside in the r15 orbital the bond order is 2 - 2 2 0. Molecular orbital theory predicts that a molecule with a bond order of zero will not exist and He2 in fact does not exist. We can do similar analyses of the molecules Li2 and Be2. The Li and Be atoms have groundstate electron configurations of Heps1 and He 2s2 respectively. The 2s atomic orbitals also combine to form the corresponding a and a molecular orbitals. Figure shows the molecular orbital diagrams and bond orders for Li2 and Be2. As predicted by molecular orbital theory Li2 with a bond order of 1 is a stable molecule whereas Be2 with a bond order of 0 does not exist. 77 Molecular Orbitals In order to consider diatomic molecules beyond Be2 we must also consider the combination of p atomic orbitals. Like s orbitals p orbitals combine both constructively to give bonding molecular orbitals that are lower in energy than the original atomic orbitals and destructively to give antibonding molecular orbitals that are higher in energy than the original atomic orbitals. However the orientations of px py and p7 orbitals give rise to two different types of molecular orbitals a molecular orbitals in which the regions of electron density in the bonding and antibonding molecular orbitals lie along the internuclear axis and tt .
