This element is added to nitrogen and oxygen to form trimix.
This element is added to nitrogen and oxygen to form trimix. A dimer of this element has the weakest known chemical bond, though that dimer shouldn't exist according to molecular orbital theory due to two electrons in the sigma sub 1s star orbital. The nuclei of one isotope of this element can form Cooper pairs at low temperatures in a mechanism described by Lee and Osherroff; this was an early example of superfluidity. Because it is produced by the proton-proton chain, it is the second most abundant element in the universe. For 10 points, name this element with atomic number two.
This element is added to nitrogen and oxygen to form trimix.
{helium} [or {He}]