What is a general rule for achieving thermodynamic stability?

It should be as low in energy as possible.

An easy analogy would presumably be coffee... you may want some coffee to get you awake in the morning, but you don't want THAT much coffee. You don't want to be hyper, you just want to be awake.

To accomplish that after-the-fact, YOU personally might take a sleeping pill, or exercise your energy away so that you tire yourself out to your regular awake self.

Atoms (and molecules) want to be stable, and they'll do that by being as low in energy as possible, however that's accomplished.

Here are common ways for atoms:

• Filling up the outer "shell" with electrons, however many that may be. That makes the atom less likely to react, making it more stable.

For examples, see the noble gases, because they don't often react.

• Reducing the amount of electron repulsions.

This can be accomplished in three ways. Sometimes one way is sufficient, sometimes two are needed. Sometimes one trumps the other two.

Method #(1)# is by having an electron in a lower-energy orbital instead of in a higher-energy orbital. That makes the atom more stable.

Method #(2)# is unpairing an electron, which follows Hund's Rule: more unpaired electrons = more stable (usually).

For example, chromium has an electron configuration of #[Ar]3d^5 4s^1#, rather than #[Ar] 3d^4 4s^2#, through methods #(1)# and #(2)#.

Method #(3)# is sometimes, it may be worth putting the electron into a bigger orbital. More space, less repulsions. That makes the atom more stable.

An example is tungsten. Unlike chromium, it is fine with #[Xe] 5d^4 6s^2# (instead of #[Xe] 5d^5 6s^1#), and this is most likely because method #(3)# helped more than method #(2)# and method #(1)# combined.