Nothing could come between them, a love story of two electrons

Our previous blog post taught us about why electrons form electron clouds around a nucleus. It turns out that the position of an electron is sufficiently unknown that it occupies an electron cloud around its atom rather than occupying a specific point. If we know that there will be multiple electrons around a nucleus, how do they interact with each other?

There are two ways that electrons behave and that depends exactly how that electron is configured. Electrons are most happy when they pair up in so-called electron pairs. An electron has a property called spin which can either be up or down. Electron pairs always have one electron that is spin up and another that is spin down. This spin pairing stabilizes the two electrons and makes them much more stable than solo electrons.

In this chart below you can see that if the two spins are the same orientation, that they become more and more repulsive as the distance between the two electrons decreases (red line). For spins of opposite orientations (blue line), they initially have a repulsion toward each other, but as the distance shrinks to zero, the repulsive energy also goes to zero. This tells us that electrons of opposite spins like to occupy the same position, which is what they are doing in a molecular orbital.

Electrons that have the same spin (red) get more and more repulsive as they get closer to each other. Electrons with opposite spins (blue) initially start off repulsive but once they get over a small hump in energy their repulsive energy goes to zero.

Molecular configurations can exist with unpaired electrons and these are called radicals because they are so reactive. If we take two CH3 radicals and put them in the same container, they have to get over the repulsive hump that we see labeled as Eb, but once they do, they will rapidly combine to form H3C-CH3 or ethane because it is most stable for those two electrons to occupying the same place.

Two methyl radicals oriented toward each other and about to create ethane. We can represent an unpaired electron with a dot.

So this shows us that unpaired electrons are reactive, but what happens once the electrons are paired into a stable structure? In the case where a reaction does not occur, when you bring two molecules together there is a repulsion that occurs between the two sets of electron clouds that gets more and more intense as they get closer.

Electrons repel each other due to coulombic repulsion. You have to push so hard to overcome it that it isn’t feasible under normal circumstances.

If you look at this chart, we can see the distance that the nucleis are, so as you move to the left on the chart, you are bringing them closer together. You can see that the repulsion goes up. It is this repulsion that prohibits two objects that come close to each other from passing through each other.

Of course this repulsive force isn’t infinite and you can see on the chart that if you really try hard enough you eventually can bring two nuclei together and that the energy required actually flips to be negative. This tells us at a certain point you actually get energy back out but at this point you are actually performing nuclear fusion on the atoms in question!

Since the energy required to compress atoms that hard together is so immensely intense, this is never a regime that we encounter in everyday life. It is for this reason that you remain firmly planted in your chair and don’t have to worry about falling to the center of the earth.

Originally published at www.pocketscholar.com.