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2science t1_ivrp6fv wrote

Reply to comment by mfb- in Why do neutrons tend to interact more readily with light nuclei rather than heavy ones (i.e., why is water a better neutron shield than lead)? by Calgaris_Rex

How is that water has higher density of hydrogen if it contains also oxygen in it?

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ScootysDad t1_ivsvi4c wrote

Surprisingly, water is 14x denser than liquid hydrogen. The bonding angle of the H-O-H makes the O slightly positive and the H slightly negative which then tends to attract another H from the other H-O-H and holds it closer (surface tension). Even at cryogenic level water will have more H than liquid H.

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2science t1_ivsvpp7 wrote

I wonder whether water is considered denser than liquid hydrogen because it's heavier.

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ccdy t1_ivtgbbo wrote

Water has a hydrogen atom number density of 111 mol/L, while that of liquid hydrogen is 70.3 mol/L. No, it's not just due to water having a higher molecular weight.

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Calixare t1_ivtxzzh wrote

So, BeH2 and liquid NH3 will be slightly better. Of course, applying water is much easier.

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ScootysDad t1_ivvp1bu wrote

The BeH2 molecule has a very linear structure with the H atoms bonded "inline" so they cannot be as densely packed as H2O. Further, it's actually a solid and form a kind of crystalline (lots of open space). NH3 (amonia) bonding is something vague for me but I think it has something to do with the electron orbital in O that give is a higher charge than N thus allowing it to be more attractive than N in that configuration. I don't think it comes even close.

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Calixare t1_ivwo620 wrote

But they do. Respectively, 118 and 120 mol H per L.

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thewiselumpofcoal t1_ivswjye wrote

Liquid hydrogen has a density of 71 kg/m^3, water is at roughly a thousand, so it's 14 times denser. They both have 2 hydrogen atoms per molecule and water molecules are 9 times more heavy than H2, so in water there's just more molecules per unit of volume.

Between water molecules there's a much stronger attractive force than between hydrogen molecules, so they are pulled much closer together. So much that liquid water is famously even denser than ice.

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