• Choose a category
• All
• Classic-Fiction

By Root 323 0

need to make it much colder. It is possible to put liquid O2 in a test tube hanging from a string. If you bring a magnet up to the test tube, you can actually pull the test tube around with the magnet. The electron spins (little bar magnets) of O2 are lined up by the magnetic field of the external macroscopic magnet. These lined-up little bar magnets add together to make the liquid O2 magnetic, and the test tube of O2 is attracted to the external magnet. The correct prediction that O2 is paramagnetic from the analysis we have done using the MO energy level diagram is remarkable. The magnetic moment of O2 is strictly a quantum effect, and our prediction that O2 is paramagnetic comes from the application of Hund’s Rule. By following some rules, we drew some lines to represent the energy levels. Then following more rules, we put some arrows up and down on the energy level lines (we put in the electrons). Using these lines and arrows we can say that the oxygen molecule is magnetic while the fluorine and nitrogen molecules are not.

THE NITROGEN MOLECULE

Figure 13.9 shows the filled MO energy level diagram for nitrogen, N2. The nitrogen atom is immediately to the left of oxygen on the Periodic Table. Note that there is a switch in the ordering of the bonding MOs derived from the p electrons. Detailed quantum mechanical calculations give the ordering and the actual energies of the MO energy levels. For nitrogen, the ordering is different than for O2 and F2. The nitrogen atom has seven electrons, so N2 has a total of 14 electrons. As with F2 and O2, the 1s and 2s electrons do not play a role in bonding because they fill both the bonding and antibonding MOs. Filling these MOs uses up eight of the 14 electrons. The other six electrons fill the three bonding MOs, one σ MO and two π MOs. There are no electrons in the π antibonding MOs or the σ antibonding MO formed from the pz orbitals. Therefore, N2 has a bond order of 3, a triple bond. A triple bond will be stronger and shorter than a double or single bond. Note that there are no unpaired electrons in N2. N2 is not paramagnetic. At low temperature, below—196° C (—320° F), N2 is a liquid. However, you cannot move a test tube of liquid N2 with a magnet because it does not have unpaired spins.

SINGLE, DOUBLE, AND TRIPLE BONDS

In discussing bonding in Chapter 11 based on an atom’s position in the Periodic Table, we used the idea that an atom would form covalent bonds to share electrons so that it could reach the noble gas configuration. For the second row elements we have been discussing here, nitrogen, oxygen, and fluorine, the noble gas is neon. We said that F, which is one electron from the Ne configuration, would share one electron with another atom. O, which is two electrons from the Ne configuration, would share two electrons, and N, which is three electrons from Ne, would share three electrons. Here we saw that F2 has a single bond, O2 has a double bond, and N2 has a triple bond. The type of bond between atoms, that is single, double, or triple, can be indicated as F—F, O = O, and N≡N. It is through the bonds that atoms share electrons. A covalent bond is an electron pair sharing bond. A double bond shares two pairs of electrons. A triple bond shares three pairs of electrons. When a bonding MO is exactly canceled by its corresponding antibonding MO, the electrons are not really shared by the atoms. They are in molecular orbitals, but the bonding MO produces constructive interference of the probability amplitude waves and the antibonding MO generates destructive interference. These cancel each other. The electrons in this situation are referred to as lone pairs. These are pairs of electrons that do not contribute to bonding. It is the F2 single bond, a shared pair of electrons, that provides the extra electron each F atom needs to give it the Ne configuration. In O2, the double bond (sharing of two pairs of electrons) provides two extra electrons for each O atom to give them the Ne configuration. In N2, the triple bond (sharing of three pairs of electrons) provides