Homonuclear diatomic description

In the molecular orbital description of homonuclear diatomic molecules, we first build all possible molecular orbitals from the available valence-shell atomic orbitals. Then we accommodate the valence electrons in molecular orbitals by using the same procedure we used in the building-up principle for atoms (Section 1.13). That is,... 

Although the most naive form of valence-bond and Lewis-structure theory would not predict the paramagnetism of O2, the VB-like NBO donor-acceptor perspective allows us to develop an alternative localized picture of general wavefunctions, including those of MO type. Let us therefore seek to develop a general NBO-based configurational picture of homonuclear diatomics to complement the usual MO description. 

The description of bonding for homonuclear diatomic molecules (molecules containing two identical atoms) is qualitatively correct for heteronuclear diatomics (molecules with two different atoms). Thus we can predict that CO, which has 10 valence electrons and is isoelectronic with N2, has a bond order of... 

Apply the Aufbau Principle to find molecular orbital descriptions for homonuclear diatomic molecules and ions... 

A homonuclear diatomic molecule is one in which both nuclei are the same, for example H2 and N2. In the first row of the Periodic Table, H2 is the only example. From the second row we have N2, 02 and F2, which are stable under normal conditions of temperature and pressure. We looked at N2 in the previous Section. Here we shall consider the molecular orbital description of 02, and use it as an example of how we can use the theory to explain and/or predict properties of molecules. 

Homonuclear diatomic moiecuies 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—L12, 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(l) and 2A(2), and then at those from the p block. Groups 3A(13) through 8A(18). 

Sketch the behavior of the bond order and the bond length for the homonuclear diatomic series Li2-Ne2, based on a molecular orbital description. 

Fig. I. Description and denomirmtion of the investigated approaches by a homonuclear diatomic molecule of a homonuclear metal cluster involving I. 2 or 3 atoms.

The treatment of heteronuclear diatomic molecules is similar to that for homonuclear diatomic molecules. We first consider the MO description. 

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. 

The principles we have used in developing an MO description of homonuclear diatomic molecules can be extended to heteronuclear diatomic molecules—those in which the two atoms in the molecule are not the same—and we conclude this section with a fascinating heteronuclear diatomic molecule—nitric oxide, NO. 

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 param pietism, 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. The molecular orbitals of heteronuclear diatomic molecules are often closely rdated to those of homonuclear diatomic molecules. 

Given this description, we can now examine the electron configuration of the second-period homonuclear diatomic molecules ... 

Just as we treated the bonding in H2 by using molecular orbital theory, we can consider the MO description of otiier diatomic molecules. Initially we will restrict our discussion to homonuclear diatomic molecules (those composed of two identical atoms) of elements in flie second row of the periodic table. As we will see, die procedure for determining the distribution of electrons in these molecules closely follows the one we used for H2. 

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