Uniqueness of Helium

Helium. He, is used to fill balloons, in lasers for eye surgery, as a cooling agent in nuclear reactors, and as a pressurizing agent for liquid fuel rockets in space exploration.

Helium has a number of characteristics that make it special. It is the most stable of all the elements and has the lowest boiling point. It becomes a fluid at temperatures close to absolute zero (0K) while most other materials are solid. In fact, helium is a liquid even at 0K and becomes a solid only under high pressure, and, helium is the only substance to exhibit superfluidity.

Of all the elements, helium is closest to the ideal gas. Two helium atoms form the weakest bound diatomic molecule, or dimer. All the properties of temperature, a measure of the kinetic energy of particles in matter, can be modeled if the force acting between a pair of helium atoms is known.
University of Delaware scientists have now predicted that the average separation between the helium atoms is 47 angstroms, compared to a typical bond length of 1 angstrom (one ten billionth of a meter or 0.0001 micron), and that the binding energy is 6,790 times smaller than the potential depth.

Reference:
M. Przybytek, W. Cencek, J. Komasa, G. %u0141ach, B. Jeziorski, K. Szalewicz. Relativistic and Quantum Electrodynamics Effects in the Helium Pair Potential. Physical Review Letters, 2010; 104 (18): 183003 DOI: 10.1103/PhysRevLett.104.183003