Period 2 diatomic molecules

In considering the MO description of diatomic molecules other than H2, we will initially restrict our discussion to homonuclear diatomic molecules (those composed of two identical atoms) of period 2 elements. 

Period 2 atoms have valence 2s and 2p orbitals, and we need to consider how they interact to form MOs. The following rules summarize some of the guiding principles for the formation of MOs and for how they are populated by electrons  

The number of MOs formed equals the number of atomic orbitals combined. 

Atomic orbitals combine most effectively with other atomic orbitals of similar energy. 

The effectiveness with which two atomic orbitals combine is proportional to their overlap. That is, as the overlap increases, the energy of the bonding MO is lowered and the energy of the antibonding MO is raised. 

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Deduce the ground-state electron configurations of Period 2 diatomic molecules (Toolbox 3.2 and Example 3.7). 

Before we can consider the remaining period 2 diatomic molecules, we must look at the MOs that result from combining 2p atomic orbitals. The interactions between p orbitals are shown in FIGURE 9.36, where we have arbitrarily chosen the internuclear axis to be the z-axis. The 2p orbitals face each other head to head. Just as with s orbitals, we can combine 2p orbitals in two ways. One combination concentrates electron density between the nuclei and is, therefore, a bonding molecular orbitaL The other combination excludes electron density from the bonding r on and so is an antibonding molecular orbital. In both MOs the electron density lies along the internuclear axis, so they are cr molecular orbitals Cjp and tr p. 

A FIGURE 9.43 Molecular orbital electron configurations and some experimental data for period 2 diatomic molecules. 

The molecular orbital description of period 2 diatomic molecules leads to bond orders in accord with the Lewis structures of these molecules. Further, the model predicts correctly that O2 should exhibit paramagnetism, which leads to attraction of a molecule into a magnetic field due to the influence of unpaired electrons. Molecules in which all the electrons are paired exhibit diamagnetism, which leads to weak repulsion from a magnetic field. 

PERIOD 2 DIATOMIC MOLECULES We extend the concepts of molecular orbital theory to construct energy-level diagrams for second-row diatomic molecules. 

Molecular Orbitals and Period 2 Diatomic Molecules (sections 9.7 and 9.8)... 

The next homonuclear molecule composed of second row elements is B2, which has six total valence electrons to accommodate. We can approximate the next higher energy molecular orbitals for B2 and the rest of the period 2 diatomic molecules as linear... 

Draw an energy diagram for the molecular orbitals of period 2 diatomic molecules. Show the difference in ordering for B2, C2, and N2 compared to O2, F2, and Ne2-... 

Why does the energy ordering of the molecular orbitals of the period 2 diatomic molecules change in going from N2 to O2 ... 

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