Which molecular orbital is greatest in energy

We can now fill the orbitals, beginning with the one that is lowest in energy. Each fluorine has 7 valence electrons, so there are a total of 14 valence electrons in the F 2 molecule. The difference in energy between the 2 s and 2 p atomic orbitals increases from Li 2 to F 2 due to increasing nuclear charge and poor screening of the 2 s electrons by electrons in the 2 p subshell.

The bonding interaction between the 2 s orbital on one atom and the 2 p z orbital on the other is most important when the two orbitals have similar energies. Experimentally, the energy gap between the ns and np atomic orbitals increases as the nuclear charge increases Figure 9.

Use a qualitative molecular orbital energy-level diagram to predict the electron configuration, the bond order, and the number of unpaired electrons in S 2 , a bright blue gas at high temperatures. Asked for: molecular orbital energy-level diagram, bond order, and number of unpaired electrons. A Sulfur has a [Ne]3 s 2 3 p 4 valence electron configuration. To create a molecular orbital energy-level diagram similar to those in Figures 9.

B The molecular orbital energy-level diagram is as follows:. Each sulfur atom contributes 6 valence electrons, for a total of 12 valence electrons. Although many combinations of atomic orbitals form molecular orbitals, we will discuss only one other interaction: an ns atomic orbital on one atom with an np z atomic orbital on another.

As shown in Figure 9. Molecular Orbitals Formed from ns Orbitals The molecular orbitals diagrams formatted for the dihydrogen species are similar to the diagrams to any homonuclear diatomic molecule with two identical alkali metal atoms Li 2 and Cs 2 , for example is shown in part a in Figure 9. Given: chemical species Asked for: molecular orbital energy-level diagram, valence electron configuration, bond order, and stability Strategy Combine the two sodium valence atomic orbitals to produce bonding and antibonding molecular orbitals.

Draw the molecular orbital energy-level diagram for this system. Fill the molecular orbitals in the energy-level diagram beginning with the orbital with the lowest energy. Calculate the bond order and predict whether the species is stable.

Solution A Because sodium has a [Ne]3 s 1 electron configuration, the molecular orbital energy-level diagram is qualitatively identical to the diagram for the interaction of two 1 s atomic orbitals. Molecular Orbitals Formed from n p Orbitals Atomic orbitals other than ns orbitals can also interact to form molecular orbitals.

Energies for Homonuclear Diatomic Molecules We now describe examples of systems involving period 2 homonuclear diatomic molecules, such as N 2 , O 2 , and F 2. When we draw a molecular orbital diagram for a molecule, there are four key points to remember: The number of molecular orbitals produced is the same as the number of atomic orbitals used to create them. As the overlap between two atomic orbitals increases, the difference in energy between the resulting bonding and antibonding molecular orbitals increases.

These values correspond to the valence bond model, so a bond order of 1 is equal to a single bond, and 2 is equal to a double bond. A value of zero means that there is no bond present, and the atoms exist separately. Generating molecular orbitals of molecules more complex than hydrogen using the LCAO method requires following a few additional guidelines:. Similarly 2 s atomic orbitals combine, giving a bonding orbital and an antibonding orbital, which are filled with the remaining valence electrons starting from the bottom up.

The atomic orbitals that combine are of similar energy levels; a 1s orbital does not combine with one of the 2 s orbitals. To determine the molecular orbitals of many other molecules, we need to examine how p orbitals combine to give molecular orbitals.

The p orbitals can overlap in two ways: head-to-head or sideways. The energy diagram we have just generated fits experimentally with O 2 , F 2 , and Ne 2 , but does not fit for B 2 , C 2 , and N 2. However, the molecular orbital diagram we see in Figure 9. We can focus further on two very important types of molecular orbitals: the highest occupied molecular orbital HOMO and the lowest unoccupied molecular orbital LUMO , also referred to collectively as the frontier molecular orbitals Figure 9.

As their names imply, the HOMO is the molecular orbital that has the highest energy and contains electrons, while the LUMO is the lowest energy molecular orbital that does not contain electrons. This type of transition can be observed in ultraviolet-visible UV-Vis radiation spectroscopy experiments.

Therefore, understanding frontier molecular orbital energy levels can provide chemists with a great deal of insight in the areas of molecular spectroscopy and reactivity. Skip to content Chapter 9. They are buried a little deeper in the atom, and they don't play a very important role in bonding. Ignoring the core electrons is pretty common; if you recall, in atomic electron configurations we might write [He]2s 2 2p 4 instead of 1s 2 2s 2 2p 4 for oxygen; we were ignoring the core.

When we drew Lewis structures, we gave oxygen six electrons, rather than eight; we were ignoring the core. In the context of MO, suppose we do have 2s electrons.

That must mean that each atom has two 1s electrons of its own, for a total of four. The effect of both those combinations being occupied is to cancel out the bonding; those two pairs of electrons remain non-bonding. So we can ignore them and we aren't really missing anything. The 2s orbitals aren't the only ones in the second shell. There are also 2p orbitals. Remember, there are a couple of very different ways in which p orbitals can combine with each other, depending upon which axis they lie.

If they do not lie parallel to each other -- that is, if they are prependicular to each other, such as a p x and a p y -- then they cannot interact with each other at all. The p z on one atom could interact with the p z on the other atom, however, because they are parallel to each other. Usually, we define the z axis as lying along the line between the two atoms we are looking at. Two p z orbitals would lie along that axis, each with a lobe extending into the space between the atoms, and each with another lobe extending away, in the other direction.

There are also those p orbitals that do not lie along the bond axis, or the axis between the two atoms.

The p x orbitals are perpendicular to the p z orbitals we just looked at, and therefore perpendicular to the axis between the bonds. However, they are still parallel to each other, and they can still form combinations.