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Ralph Mowery

In article ,
says...

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 atoms are lighter than He, on average.

Hydrogen is also explosive or will burn. That is another reason not to
just spray it out for leak detection.

I think that hydrogen may be the smallest atom, but they often join in
pairs to make up a larger molicule. Some other atoms that are normally
gas do the same thing.