John Carlos Baez on Nostr: Even though helium is the least reactive of the noble gases, you can make a molecule ...
Even though helium is the least reactive of the noble gases, you can make a molecule of two helium atoms! Yes, He₂ is a thing! It's called 'dihelium' or the 'helium dimer'.
But the two helium atoms aren't held together by a bond. Instead, they're held together by a much weaker force: the van der Waals force. So this isn't an ordinary molecule. It's huge! The distance between nuclei is 70 times bigger than it is for H₂. And it's very loosely held together, so you'll only see it at extremely low temperatures.
It's called a 'van der Waals molecule'.
But what's the van der Waals force? It's actually a name for a few different forces that exist between electrically neutral objects. For dihelium, the most important one is shown in this gif.
Start with two neutral atoms. Each is a nucleus surrounded by electrons. If they start out spherically symmetric, like helium, and far enough apart that their electron clouds don't overlap, there will be no electrostatic force between them.
But wait - the electrons are randomly moving around! Sometimes they move to make each atom into a 'dipole', with the nucleus on one side, and the electrons over on the other side. If these two dipoles point the same way, this lowers the energy - so it will tend to happen. And now the two atoms attract... since two dipoles pointing the same way attract.
But this effect is small. The gif vastly exaggerates how much the electrons move to one side of each proton. And this van der Waals force dies off like 1/𝑟⁶. So we have two objects held together by a weak force... so they will orbit slowly, and far apart, so that the centrifugal force is equally small!
(1/2)
But the two helium atoms aren't held together by a bond. Instead, they're held together by a much weaker force: the van der Waals force. So this isn't an ordinary molecule. It's huge! The distance between nuclei is 70 times bigger than it is for H₂. And it's very loosely held together, so you'll only see it at extremely low temperatures.
It's called a 'van der Waals molecule'.
But what's the van der Waals force? It's actually a name for a few different forces that exist between electrically neutral objects. For dihelium, the most important one is shown in this gif.
Start with two neutral atoms. Each is a nucleus surrounded by electrons. If they start out spherically symmetric, like helium, and far enough apart that their electron clouds don't overlap, there will be no electrostatic force between them.
But wait - the electrons are randomly moving around! Sometimes they move to make each atom into a 'dipole', with the nucleus on one side, and the electrons over on the other side. If these two dipoles point the same way, this lowers the energy - so it will tend to happen. And now the two atoms attract... since two dipoles pointing the same way attract.
But this effect is small. The gif vastly exaggerates how much the electrons move to one side of each proton. And this van der Waals force dies off like 1/𝑟⁶. So we have two objects held together by a weak force... so they will orbit slowly, and far apart, so that the centrifugal force is equally small!
(1/2)