Exceptions are the rule?!

So each year I am inspired by something interesting I’ve read or experienced to write about on my home page…this year is no exception (no pun intended). Some of you may already know, and the rest of you will soon know, about the Aufbau Principle. It helps us to predict electron configurations, which are really just maps of where the electrons lie in an atom. They indicate the shell (distance from the nucleus) and the orbital (shape of the area that the electrons occupy) where the electrons are located. We care about this because of a few reasons but they help us identify the valence electrons that do all the cool bonding stuff. This has been taught to me for years in the same way…1s2 2s2 2p6 3s2 etc. We fill inner shells (like 1 and then 2 and then 3) and we fill s before p and when we get to the d orbitals things get funky. I learned (and I’ve always taught) that we fill the s shell before the d shell because the d shell is higher in energy and is therefore less favorable to fill. For scandium you would have a 1s2 2s2 2p6 3s2 3p6 4s2 3d1 configuration, and the 4s fills first followed by the 3d…right?! Yeah, maybe not.

Turns out we can do (well not me but other scientists) can do photoemission spectroscopy (PES) which is where you shoot high energy photons at an atom to eject electrons. Why would we do this? We learn about inner core electrons that are difficult to study, and it’s just cool, so why not? Well it turns out the 4s IS actually at a higher energy than the 3d…what?! Aaaannnddd the 4s is not necessary filled first, but electrons are promoted to the 4s because of all the complicated electron electron repulsions in the 3d as well as the nuclear attractions. Aaaaannndd the so-called “exceptions” where the 4s isn’t filled but the 3d is happy in a half filled (5 electons) or fully filled state (10 electrons, like Copper) might actually be the “rule” and everyone else is the exception!

Okay so maybe you followed my diatribe and maybe you were completely lost and uninterested but when you’ve been thinking of something happening one way for 14 years it’s really cool when science is like “just kidding, I’m so much more complex than you thought”. Science is ever evolving and it always laughs at us silly scientists for trying to make rules for everything.

I guess if there’s a life lesson to learn here it’s that if you think you do something weird and you are the “exception”, maybe everyone else are weirdos and you’re just smarter than the rest of us…joke’s on us. Happy 2013-2014 school year chem nerds, it’s gonna be awesome!

(Wanna read more about this science? Click here)

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