Molecular helium

Molecular helium is a form (allotrope) of the chemical element helium that contains chemical bonds. Helium is a noble gas and normally exists as a single atom, written just , but it can form molecules in special conditions.

 Dimeric  
Two helium atoms can make a molecule that is bound by the van der Waals force. This  molecule is only stable at extremely low temperatures.

 is the largest known diatomic molecule (molecule made of two atoms), with a distance between the helium atoms of 52 ångströms.

  excimer 
A different type of  exists when the helium atoms are in an excited state. A molecule that is more stable in an excited state than the ground state is called an excimer, and is written with a * symbol.  was the first excimer discovered by scientists. 

 Trimeric  
Trimeric helium is made of three atoms. Like dimeric helium, it is bound by the van der Waals force. The  trimer is larger but more stable than .

 Sources 

Category:Helium compounds