On Monday, May 20, 2019 at 3:46:16 PM UTC-7, Cursitor Doom wrote:
On Mon, 20 May 2019 08:11:31 +0100, Mike Coon wrote:

In article ,
says...

Hydrogen atoms are really really small.
Trying to keep hydrogen in or out is always problematic.

In practice you get molecules on the two-fer principle which are much
bigger. Helium is bad too because then the atoms come at you singly...

May seem counter-intuitive, but hydrogen is not actually the smallest
atom. Heliums come singly alright, which is one of the reasons, along
with small size, they're used for this kind of leak testing.


Good point; also, the H2 molecule has two 1S-like electrons, which is
very similar to a Helium atom, and just about as slippery.
H2 molecules are lighter than He atoms, on average.