
Helium has
an atomic number of two.
The Big Bang Theory Season 7, Episode 3

Hey, in your super-cooled
HELIUM experiments,
The Big Bang Theory Season 9, Episode 16

Be even better if he swapped out
the HELIUM for xenon.
The Big Bang Theory Season 11, Episode 8

the abbreviation for HELIUM.
The Big Bang Theory Season 7, Episode 3