Pure hydrogen is less dense than pure helium. Why does it have a higher melting point?
In: Chemistry
Because density doesn’t really change anything – it’s about how much the molecules are attracted to each other. Helium is a noble gas, with its whole outer electron shell full, and so it has very little intermolecular attraction – less than hydrogen.
Since it’s Eli5, I won’t be answering like the other two and I’ll keep it a bit simple.
On achieving noble gas configuration, the size of the orbitals explodes a bit. In liquid state, this exploded size means less interatomic attraction (so Helium attracts Helium less than hydrogen attracts hydrogen). That’s because of the complete outer duplet. Electrons repel electrons and that’s why atomising helium is much easier. Density of helium is higher because it shows a 4 times increase in mass for a small increase in size. Atomic attractions are not affected by mass.
The phases of a material (solid liquid gas) don’t just depend on density, but also on the forces or links between the atoms (i.e. on the orbitals / chemistry of the atoms).
So helium atoms generally don’t react with other helium atoms, whereas hydrogen atoms usually form covalent bonds (form hydrogen molecules, H2). So these bonds require extra energy to “break apart” and get a material from, say, solid to liquid, or from liquid to gas.
Imagine that atoms are hyperactive kids and like to run around and play. This is what atoms are like in gas state, they roam and expand to the edge of how you possibly limit them. To turn them into liquid you need the atoms to stick close to each other and stop moving so damn much. So how do you make the kids stick together instead of running around? You can grab them one by one, but they’ll just keep slipping away and run. To guarantee it, you need to force them, and a way to force them is to really lower the room’s temperature. When the kids will feel that it’s cold, they’ll realize at some point that a good way to stay warm is to stick really close to each other and still move every now and then and switch places, but not move away again because it’s too cold. This is what atoms are like in liquid state.
Now imagine that those kids are really snobby kids that don’t get along well with each other and prefer to keep their safe distance. Like some stereotypical noble fancy kids. Noble gas atoms. If you use the same method as in the previous paragraph, they might still get closer together but you’re gonna need to make the room much, much colder to convince them that their need to stick together is stronger than their will to be on their own.
Helium is a noble gas, and hydrogen is not. To convince the helium, snobby atoms to get along and stick together, you’re gonna need a far more dire situation that you would in the case of the hydrogen, non-snobby atoms.
**Disclaimer: This is a metaphor only. I strongly urge you not to physically freeze your kids when they’re energetic.**
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