Last times

There is, they say, a last time for everything. It is certainly true for me this (last) academic year. There are many last times.  I have graded the last quantum midterm, and today made up the last problem set for the class. Last week in intro chemistry, I taught VSEPR theory for the last time. It takes practice to be able to visualize the 3D structures of the molecules from the sketches that chemists make, and so I give them all a simple molecular model kit they can carry around. It all fits neatly into a centrifuge tube.

I bring all the pieces to class and have students assemble their own kits. This time when I went to put the extra bits and bobs away, tossing the little signs — “Take 3” — into the bag I realized with a start that I would not need these things again. I could just toss the signs and give away the 50 extra octahedral centers. Except I couldn’t. It seemed too…real. Too final.

Somehow the last things are less real when there is not actual stuff attached to them. Literally weightier, they feel metaphorically heavier, too. I repacked the bag and put it back in the cupboard. 

Maybe it is the time of year, not just my imminent retirement. The days are shorter, the sun struggles to climb high in the sky, and the readings at Mass are circling around the end times.

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VSEPR stands for valence shell electron pair repulsion and basically says that bonds around an atom will arrange themselves to be as far apart as they can because the electrons that make up the bonds repel. So if an atom has two other atoms bonded to it, they will be in a line (CO₂ is an example), 180^o apart. There are a dozen or so patterns based on this theory.

Centrifuge tubes are great for storing small amounts of stuff and you don’t have to be a scientist to buy them. They don’t usually leak, either.