4/9/20215.4 Molecular Orbital Theory - Chemistry: Atoms First 2e | OpenStax1/18Learning ObjectivesBy the end of this section, you will be able to:Outline the basic quantum-mechanical approach to deriving molecular orbitals from atomic orbitalsDescribe traits of bonding and antibonding molecular orbitalsCalculate bond orders based on molecular electron configurationsWrite molecular electron configurations for first- and second-row diatomic moleculesRelate these electron configurations to the molecules’ stabilities and magnetic propertiesFor almost every covalent molecule that exists, we can now draw the Lewis structure, predict the electron-pair geometry, predict the molecular geometry, and come close to predicting bond angles. However, one ofthe most important molecules we know, the oxygen molecule O, presents a problem with respect to itsLewis structure. We would write the following Lewis structure for O:This electronic structure adheres to all the rules governing Lewis theory. There is an O=O double bond, andeach oxygen atom has eight electrons around it. However, this picture is at odds with the magnetic behaviorof oxygen. By itself, Ois not magnetic, but it is attracted to magnetic fields. Thus, when we pour liquidoxygen past a strong magnet, it collects between the poles of the magnet and defies gravity, as inFigure5.1. Such attraction to a magnetic field is calledparamagnetism, and it arises in molecules that haveunpaired electrons. And yet, the Lewis structure of Oindicates that all electrons are paired. How do weaccount for this discrepancy?Magnetic susceptibility measures the force experienced by a substance in a magnetic field. When wecompare the weight of a sample to the weight measured in a magnetic field (Figure 5.27), paramagneticsamples that are attracted to the magnet will appear heavier because of the force exerted by the magneticfield. We can calculate the number of unpaired electrons based on the increase in weight.2222
4/9/20215.4 Molecular Orbital Theory - Chemistry: Atoms First 2e | OpenStax2/18Figure 5.27A Gouy balance compares the mass of a sample in the presence of a magnetic field with the mass with theelectromagnet turned off to determine the number of unpaired electrons in a sample.Experiments show that each Omolecule has two unpaired electrons. The Lewis-structure model does notpredict the presence of these two unpaired electrons. Unlike oxygen, the apparent weight of mostmolecules decreases slightly in the presence of an inhomogeneous magnetic field. Materials in which all ofthe electrons are paired arediamagneticand weakly repel a magnetic field. Paramagnetic and diamagneticmaterials do not act as permanent magnets. Only in the presence of an applied magnetic field do theydemonstrate attraction or repulsion.