G. H. Saban and T. F. Moran Georgia lnst~tuteof Technology Atlanta, 30332
A Simple Demonstration of Oz Paramagnetism A macroscopica/~yobservable difference between VB and MO approaches to bonding theory
For many students, the initial exposure to chemical bonding is from the viewpoint of the valence bond (VB) theory. During the first year of college chemistry, molecular orbital (MO) theory is usually introduced to explain some of the molecular properties that cannot be rationalized with the VB model. For example, the VB model predicts the structure of OZto be
.. .. !?=!?
where the octet rule has been satisfied and all electrons are paired. However, a simple MO approach predicts the electronic configuration of OZto be
tion in the x-direction which was consistent with that estimated from the significant forces (mavitational, friction. al, and magnetic) acting on an individual 1-mm droplet. The experiment ran for about 10 min/l of liquid nitrogen charge. As an interesting comparison, liquid nitrogen dropped through the same magnetic field (by way of a funnel drawn from half inch glass tubing) was undeflected. These observations were consistent with MO theory which predicts the electronic configuration of Nz to be the same as OZ without the unpaired electrons in the antibonding * orbitals. Since both 0 2 and Nz are good corresponding state fluid^,^ one would expect differences in magnetlc behavior to be primarily due to the paramagnetism of 0 2 which can be clearly seen by using this rather simple demonstration.
in which there are two unpaired electrons.' If this model is correct, the 0 2 species with its unpaired electrons should be paramagnetic. With this difference between the two models in mind, we have designed and used a simple apparatus (shown in Fig. 1) to demonstrate the paramagnetic behavior of oxygen. The apparatus consisted of: a 3500-gauss permanent magnet (from a surplus Navy magnetron), an aluminum tube tapped a t the bottom to accept a threaded conic section, a styrofoam reservoir (the protective packaging from a pint bottle of acid) press-fitted to the top of the tube, and a ringstand.2 Five minutes after pouring a liter of liquid Nz (bp 77°K) into the reservoir, liquid oxygen (bp 90°K) was observed to condense and drip from the conic section. The droplet trajectory was readily deflected when placed in an inhomogeneous magnetic field. We observed a 30-mm (maximum) deflec-
'Dickenon R. E., Gray, H. B. and Haight, Jr., G. P., "ChemicalPrineiples" W. A. Benjamin, Inc., New York, 1970. 2A high intensity lamp may be used to better illuminate the oxygen drops formed in the experiment. aGuggenheirn, E. A,, J. Chem. Phys. 13,253 (1945).
Paramagnetism demonstration apparatus: ( 1 ) styrofoam reservoir. (2) aluminum tube. (3) conic section. (4) magnet. and (5) ringstand with vinyl insulated clamp. The capacity of the aluminum tuba and conic section was 130 cm3. A 9 F/cm scale is presented to the right of the diagram.
Volome 50,Number 3. March 7973 / 217
