Absolutely Small - Michael D. Fayer [85]
FIGURE 13.7. The MO energy level diagram for the hypothetical molecule, Ne2. Two neon atoms have 20 electrons. There are the same number of bonding and antibonding electrons, so there is no bond. Ne2does not exist.
THE OXYGEN MOLECULE: HUND’S RULE MATTERS
One atom to the left of fluorine on the Periodic Table is oxygen. O2 is an important example that introduces a couple of new ideas. Figure 13.8 shows the MO energy level diagram filled with O2’s 16 electrons, eight from each oxygen atom. The bonding and antibonding MOs arising from the 1s and 2s orbitals are filled. These do not contribute to bonding. There are two electrons in the bonding MO and none in the corresponding antibonding MO. In addition, there are four electrons in the two π bonding MOs but only two electrons in the π antibonding MOs. The result is one σ bond and one π bond. Oxygen has a bond order of 2. It has a double bond. As will be discussed further below, a double bond is stronger and shorter than a single bond.
FIGURE 13.8. The MO energy level diagram for oxygen, O2. There is one extra pair of σ bonding electrons and one extra pair of π bonding electrons. O2has a double bond. Note the unpaired electrons in the π antibonding MOs.
O2 is the first example where Hund’s Rule comes into play and is important. Note that in filling the energy levels with electrons, the last two electrons have their spins unpaired. It is possible to have unpaired spins without violating the Pauli Principle because there are two distinct π antibonding MOs. comes from the side-to-side overlap of the two px atomic orbitals (see Figure 13.3), and , comes from the side-to-side overlap of the two py atomic orbitals. Hund’s Rule says that the electrons will go into orbitals unpaired if the result doesn’t violate the Pauli Principle and doesn’t require using a much higher energy orbital. The two π antibonding MOs have identical energy, so Hund’s Rule comes into play.
An electron has a magnetic moment. In some sense it acts like a tiny bar magnet. It has a north pole and a south pole. The term spin for the electron quantum number comes from classical mechanics. In classical mechanics, a charge distribution that is spinning has a magnetic moment. An electron is a probability amplitude wave. It has a delocalized charge distribution. It has a magnetic moment, but it is not literally spinning. That is a classical idea. Dirac, who gave us the concept of absolute size (see Chapter 2), also showed why an electron has a magnetic moment by combining quantum theory and Einstein’s Theory of Relativity. Although an electron is not actually spinning, the name stuck, and the magnetic moment of the electron is important.
When two electron spins are paired, the north pole of one little magnet is matched with the south pole of the other. The magnetic property of one electron cancels the magnetic property of the other. However, in O2, two of the electrons are not paired. Their spins point in the same direction. The result is that the O2 molecule is referred to as being paramagnetic. It will respond to a magnet. O2 is a gas at room temperature, but if you make it very cold, below -183° C (—297° F) it will become a liquid. Water above 100° C is a gas, but if you cool it below 100° C, it becomes a liquid. O2 is the same, but you