1. Helium has a near identical melting and boiling point, and the lowest of all the elements. It’s melting point is so close to absolute zero that nothing else is cold enough to freeze it, so scientists have to use specially built pressure chambers to solidify it.
2. It is stable with just two valence electrons as opposed to all other noble gases requiring eight electrons.
3. Near absolute zero, helium becomes a superfluid where it gains zero viscosity.
4. It has the highest thermal conductivity of any element which makes it an excellent coolant.
5. It is naturally non-toxic and the most chemically inert of all elements.
6. One of it’s only known ions, helium hydride, is the strongest acid ever observed.
What gives? Why is it such a wonder substance and why does it display such unique traits compared to other elements?
In: Chemistry
It’s a noble gas, all its electron shells are full, so that makes it highly non-interactive. It’s a super tiny atom, only 2 protons. The only smaller atom, hydrogen with 1 proton, has an opening for a valence electron making it very reactive.
The innermost valence shell has two electrons, and the next two are eight, but the next two are eighteen and after that it increases to 32! That’s the nature of atoms – as atoms get larger, the shells get larger. So more electrons are required to fill them.
It has the 11th highest thermal conductivity according to [this table](https://periodictable.com/Properties/A/ThermalConductivity.v.html).
Superfluidity has been demonstrated or predicted in other elements and states, albeit [under very different conditions](https://en.wikipedia.org/wiki/Superfluidity).
Helium hydride is the strongest acid because helium desperately doesn’t want to have a proton; again, a function of its noble gas/full valence shell and how small it is – the electrons are held so tightly to the nucleus that it doesn’t want to react with anything.
It is special because it is the first noble gas as well as only the second element out of all. You can easily make such a list for hydrogen as well, it as the first element has a lot of properties not found later on.
> Helium has a near identical melting and boiling point
It doesn’t freeze at standard pressure, so that is a weird statement. Also iodine has a very narrow liquid range as well, and this is not the only example.
> It’s melting point is so close to absolute zero that nothing else is cold enough to freeze it
There simply is no melting point. You need pressure. The ultimate reason is that “0 K” cannot be reached and everything always has some “basic energy”, the _zero-point energy_. For a very simplified ELI5 explanation, you can understand it as a certain temperature it cannot get below.
> It is stable with just two valence electrons as opposed to all other noble gases requiring eight electrons.
Just an effect of the first shell only admitting 2 electrons. The later ones simply hide it below other orbitals.
> Near absolute zero, helium becomes a superfluid where it gains zero viscosity.
True, but this is not unique to helium. Other substances however require different pressures. You by the way need He-4, He-3 won’t work for superfluidity as you need _bosons_.
> It has the highest thermal conductivity of any element which makes it an excellent coolant.
Under what situations? At normal temperature diamond should beat it hands-down. When superfluid then yes, but that is cheating; other superfluids also would have really high heat conductivity.
> It is naturally non-toxic
So are iron, nitrogen, and many more.
> the most chemically inert of all elements.
True. Due to its nature as smallest noble gas.
> One of it’s only known ions, helium hydride, is the strongest acid ever observed.
Helium as the leat reactive element wants to get rid of whatever you “stick” to it. In this case hydrogen, and wanting to donate hydrogen is literally what defined pH.
There are a couple reasons, depending on which question you’re asking. I’ll start with the easy ones.
2, 5, and 6 are all because that’s how the electron shells of atoms work. Helium has 2 protons, so it can only have 2 electrons (or else it would be an ion). As such it fills the 1s orbital, the lowest energy orbital, and the highest energy orbital when all lower orbitals are filled. It and hydrogen are the only elements that have the 1s orbital as the valence shell because all others have more than 2 electrons when electrically neutral. Additionally because it is electrically neutral when shells are filled, that means that it will resist reacting with anything. All noble gases are like this, but because helium is so small, it has a tighter grip on the outermost electrons than others so it’s even less reactive. If you manage to get it to react with something there is a lot of energy in there, and so you can make some crazy reactive things.
4 is wrong. Helium is a very bad conductor. Silver is the most conductive element iirc, because it is a metal with the right number of valence electrons and size to just be super conductive.
And finally the rest. Helium is made of 2 protons, 2 neutrons, and 2 electrons. That is an incredibly stable combination. All three of these things have spin. Think of a yin yang symbol, each of the energy states of everything with spin half (fermions) is one half of that, and when you get two they can fill that energy state and become super strong and stable. They lock into place. This is what’s happening below the valence shells in atoms btw and helps explain the parts I explained above as well. As a result of this, helium really doesn’t care about interacting with anything, including itself. So when you cool it, it doesn’t care that it’s getting closer to others of the same type it just happily goes floating along still. And that’s what a gas is: a type of particle that is moving fast enough that it avoids interacting with other particles of the same type, so it can’t stop moving like it is. Some things really don’t care much, like hydrogen atoms, so you need to get them pretty cold to interact like a liquid. And very very cold to interact like a solid. But hydrogen still cares a little. Helium though? Helium is head empty only move. It needs to be cooled so much that quantum effects manifest on non quantum scales before it cares what other helium atoms are doing. At that point we need to care about that spin again. Spin half particles cannot occupy the same energy state. Whole integer spin particles, bosons, can. Helium is made of 2 spin half protons and 2 spin half neutrons. This cancels out to make it spin 0 overall. When cooled enough it begins to act like a boson, a bose-einstein condensate actually, and all those normally distinct helium atoms begin to be…the same. Kind of. So when you move one of them, they all move together. They can no longer give up any energy, so can’t stop moving from friction: zero viscosity.
The important thing here is the spin. Everything has energy states, and the three fermions we care about here have spin 1/2. This means helium, with its 2/2/2 makeup, is at the lowest energy possible while having the highest and lowest shells filled for all of them. This combined with its size explains all of this.
This was a mess but hopefully it helped a little?
1. I don’t know for sure about this one, but I would bet that it’s because it’s the smallest noble gas. Interactions between atoms increase in strength with size, and helium is already non-reactive because it’s a noble gas, so together, that makes it really easy for helium atoms to split apart into a gas.
2. It’s stable at 2 valance electrons because there are only 2 possible ground states electrons can be in with 0 angular momentum. Once those two fill up, any further electrons added to an atom need to have higher energy, and there are a lot more available states with nonzero angular momentum (meaning a lot more electrons needed to fill them up and become stable)
3. Helium becoming a superfluid is related to it having 2 neutrons and 2 protons. Each of those has 1/2 spin, so adding them up makes helium have an integer spin. Particles with integer spins are called bosons, and can form a state known as a Bose-Einstein condensate where all particles are in the same quantum state. This Bose-Einstein condensation is what leads to superfluidity.
4. Thermal conductivity is heavily dependent on state, so this definitely isn’t always true.
5. Non-toxicity is more to do with human evolution than helium itself, though not being reactive probably helps with that. It being non-reactive is again due to the fact that it’s a neutral particle that has a completely full ground electron state, so gaining any electrons takes a lot of energy, and losing any takes a decent amount too.
6. Acidity is related to how easily particles release hydrogen ions into solution. Helium hydride is basically just regular helium with a single hydrogen ion hanging out nearby. The single hydrogen ion hanging out nearby means that when you stick it into water, it’ll *very quickly* ditch the hydrogen ion and just go back to being helium, which is why it’s a strong acid.
Most of these are results of helium being the smallest stable atom to completely fill its electron states, and helium being a boson.
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