Molecular Orbital Energy Diagram

A molecular orbital diagram is also known as an MO diagram. It is a tool that describes molecular bonding qualitatively in terms set by the molecular orbital theory. It gives a thematic representation of chemical bonding in general terms defined by MO theory and the linear combination of atomic orbitals or LCAO. It is a known fact that electrons are rearranged after chemical bonding. The bonding atoms combine, and their bonding orbitals form molecular orbitals. This process causes the shared electrons to realign in the newly generated molecular orbitals. 

A molecular orbital diagram can predict bond strength, the possibility of the formation of a molecule, and the electronic transitions that can take place after bond formation. However, using these to represent molecules containing more than two atoms becomes complicated. Even relatively simple molecules like methane can be difficult to express with this diagram.

History of the molecular orbital diagram

In 1928 Friedrich Hund and Robert S. Mulliken gave the qualitative Molecular Orbital theory. Vladimir Fock and Douglas Hartree gave it a mathematical method.

Fundamental concepts of the molecular orbital diagram

A molecular orbital diagram represents the energy levels of the molecular orbitals. It follows a schematic where the molecular orbital is in the middle, and the atomic orbitals are on either side, flanking it. The atomic orbitals are placed on the sides to compare energy levels. The topmost level represents the highest energy level and decreases as the orbitals descend, with the lowest having the least energy. The molecular orbitals are connected to their respective constituent atomic orbitals by dashes placed along diagonal lines. Small vertical arrows represent the electrons that fill the different atomic orbitals and the molecular orbitals according to Pauli’s Exclusion Principle. The molecular orbital diagram does not show the actual shape of the orbitals. The energetics of the bonds between diatomic molecules can be demonstrated quite well with a molecular orbital diagram. The molecular orbital diagram is easily depicted by this method. But with molecules containing multiple atoms, the diagram may omit some bonds to simplify the representation.

The molecular orbital theory states that a molecular orbital is formed when atomic orbitals overlap. This overlap is more in certain kinds of bonds than others. The type of bonds shows more overlap than the bonds. So the orbitals of sigma bonds have more energy splitting than pi bonds. Atoms that are more electronegative hold their electrons more firmly, lowering their energy levels. It means that the energy levels of atomic orbitals are related to the electronegativity of the atoms. When two bonding atoms have orbitals with comparable energy levels, the molecular orbitals are shared between the atoms. But if there is a difference in the energy levels of the bonding atoms, then the molecular orbitals formed are concentrated on one atom and the bonds formed are ionic. Another condition under which molecular orbitals will overlap is if the bonding atoms have similar symmetry.

Diatomic molecular orbital diagrams:

1.Molecular orbital diagram 

When drawing the molecular orbital diagram of the diatomic oxygen molecule, the 2s and 2p orbitals are considered. Three atomic orbitals of the 2p orbitals split and form 3 molecular orbitals, a singly degenerate sigma orbital and a doubly degenerate pi orbital. One interesting property that can be observed through a molecular orbital diagram is whether the molecule is diamagnetic or paramagnetic. When all the electrons are paired together, there exists a slight repulsion. It makes the molecule diamagnetic. On the other hand, if there are some leftover electrons after the formation of the molecule, they cause a particular attraction of the molecule towards a magnetic field, making the molecule paramagnetic. The molecular orbital diagram shows that oxygen is paramagnetic. 

2.Molecular orbital diagram

The molecular orbital diagram shows that the two molecular orbitals mix and cause energy repulsion. So the molecular orbital diagram is arranged differently from other more familiar diagrams. This mixing causes the sigma from 2p and 2s to behave in a more non-binding manner. It is the reason behind the significant change in energy in the orbital of 2p. The molecular orbital diagram also shows that the molecule is diamagnetic, and its bond order is three. Two electrons are added to the three molecular orbitals, making the bond order of dinitrogen 3.

Conclusion

The molecular bond theory is important because it explains some things that the chemical bonding valence theory cannot explain. The tools it uses are the molecular orbital diagram. These diagrams can depict the molecular orbital energetics of diatomic molecules very efficiently, as is seen by the molecular orbital diagram. However, they begin to falter when the molecule has multiple atoms. The molecular orbital diagram is a significantly helpful tool in studying chemical bonding, how the various orbits affect the bond, and how electrons interact. It ultimately leads to a better understanding of the nature of the substance.