Hydrogen molecule looks like this

image source: phelafel.technion.ac.il

I would like to thank John Baez for bringing this image to our attention. This is the hydrogen molecule (H2) not the hydrogen atom (H). Two hydrogen nuclei are located within the green areas on the image shown above. By the way, the distance between the two hydrogen nuclei is 0.734 angstrom.

In his “Diary (October 2019)” John Baez explains that:

“The color and brightness shows how likely you are to find an electron per unit volume.”

“When you freeze hydrogen at low pressure it forms a low-density crystal. Since they’re almost round and far apart, each molecule can rotate independently!”

“Since the hydrogen molecules in solid hydrogen are free to rotate, the angular momentum of each one is almost conserved!”

“Since it’s cold, we expect the lowest-energy states to predominate. The two lowest-energy states of a hydrogen molecule are those where the total angular momentum is 0 or 1, depending on whether its proton spins are opposite or aligned. If they are opposite, so the angular momentum is 0, it’s called parahydrogen, and this has the lowest energy. If they are aligned, so the total angular momentum is 1, it’s called orthohydrogen, and this has the second lowest energy.”

“So, a crystal of solid hydrogen at low pressures can be made of any sort of mixture of orthohydrogen and parahydrogen! Its properties depend subtly on how much of each. But orthohydrogen has a bit more energy so it slowly turns into the para form.”

“There are a lot of other weird things about solid hydrogen. It’s a ‘quantum crystal’, meaning the quantum uncertainty in the positions of the nuclei is significant compared to the distance between molecules. It’s also extremely compressible.”

“Only when you compress solid hydrogen a lot — like near the core of Jupiter — does it become important that the molecules aren’t completely round. Then you get different crystal phases of solid hydrogen… culminating in a metallic phase that conducts electricity!”

I really appreciate John Baez’s research and educational activities.


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