Absolutely Small - Michael D. Fayer [84]
FIGURE 13.4. Two atoms are brought together along the z axis. Pzorbitals will approach end to end; pxand pyorbitals will approach side to side.
FIGURE 13.5. The energy level diagram for two fluorine atoms, a and b, brought together to form molecular orbitals. The atomic orbital energies are on the right and left sides. The bonding (b) and antibonding (*) MO energy levels are in the center. There are σ and π MOs. The three atomic p orbitals have the same energies. These are shown as three closely spaced lines. The spacings between the levels are not to scale.
Fluorine has nine electrons. So a fluorine atom will have two electrons in the 1s orbital, two electrons in the 2s orbital, and five electrons in the three 2p orbitals. Two F atoms have a total of 18 electrons. We now need to place the 18 electrons in the proper molecular orbitals in a manner equivalent to the way we placed the electrons in the atomic orbitals when we were building up the Periodic Table in Chapter 11 and the hydrogen molecule in Chapter 12. As before, we need to follow the three rules for filling the MOs. First is the Pauli Principle, which states that at most two electrons can be in an orbital and the two must have opposite spins. Opposite spins are represented by one up arrow and one down arrow. Second, electrons are placed in the lowest energy level first, consistent with the Pauli Principle. Third is Hund’s Rule, which states that electrons will not pair their spins if possible. In F2, Hund’s Rule will not change the results of placing the electrons in the proper orbitals. When we consider oxygen, O2, it will be important.
Figure 13.6 is the MO energy level diagram for F2 with the electrons placed in the proper orbitals. The atomic orbital energy levels shown in Figure 13.5 are not included. Only the MO energy levels are shown. The first two electrons go into the σ bonding MO formed from 1s orbitals. The next two electrons go in the σ antibonding MO formed from 1s orbitals. The electrons in the bonding MO are lower in energy than the atomic orbitals of the separated atoms, but the electrons in the antibonding MO are the same amount higher in energy. Therefore, these four electrons do not contribute to bonding in F2. The next four electrons go in the σ bonding and antibonding MOs formed from the 2s atomic orbitals. Again, these do not contribute to bonding because there are two electrons in the bonding MO and two electrons in the antibonding MO.
FIGURE 13.6. The molecular orbital energy level diagram for the F2, fluorine diatomic molecule. The atomic orbital energies are not shown. Two fluorine atoms have 18 electrons. These have been placed in the orbitals following the rules discussed for atomic orbitals in Chapter 11. There is one more filled bonding MO than antibonding MO. F2has a single bond.
Now the p electrons come into play. There are a total of 10, five from each F atom. The first two go into the σ bonding MO formed from the pz atomic orbitals. Then four electrons go into the πx and πy bonding MOs. Four electrons can go into these π molecular orbitals because there are two MOs, and each can take two electrons according to the Pauli Principle. The last four electrons go into the πx and πy antibonding MOs. The four electrons in the π antibonding MOs cancel the bonding effect of the four electrons in the π bonding MOs. Therefore, the π electrons do not contribute to bonding. However, nothing cancels the two electrons in the σ bonding MO formed from the pz atomic orbitals because there are no electrons in the corresponding antibonding MO. The net result is that one pair of bonding electrons is not canceled out, so F2 has a bond order of 1 like H2. We say that F2 has a single bond, and it is a σ bond. The single covalent bond comes from having two electrons in a bonding MO. Molecular orbitals are probability amplitude waves that span the entire molecule. The atomic nuclei share these electrons.
THE NEON MOLECULE DOESN’T EXIST
We can use the same MO energy level diagram shown in Figure 13.6 to consider the hypothetical