Atoms may be thought of as assembHes of fermions only, but can behave as either fermions or bosons. Fermions, which include electrons, protons, and neutrons, are governed by Fermi-DHac statistics which forbid any two particles to occupy exactly the same quantum state and thus forbid any analogue of Bose-Einstein condensation. Bosons are governed by Bose-Einstein statistics which, at sufficiently low temperatures, allow the particles to coUect into a low energy quantum level, the so-called Bose-Einstein condensation. Quantum mechanics defines two fundamentally different types of particles bosons, which have no unpaired quantum spins, and fermions, which do have unpaired spins. Indeed, recent research on Bose-Einstein and Fermi condensation reveals that a collection of millions of atoms can enter an identical quantum state at temperatures just above 0 K and behave as a single atom, with a single wavefunction that spreads over the whole collection.
Electron orbitals extend from the nucleus to a greater or lesser extent, depending upon the chemical and physical environment in the locality of the atomic nucleus. Quantum mechanics makes it clear that no atom has a fixed size. Finally, in the future, boson condensate cavities immersed in dilute atomic Fermi condensates could serve as further system with which the effective interactions of cavities inside a Fermi gas can be studied in the lab.
Another example would be buckyballs in liquid 3He as Fermi gas. The liquid metal itself serves only as free-moving shapeable neutral background which provides the Fermi gas environment by its conductance electrons, in which the buckyballs drill the voids. Examples are C6o buckyballs immersed in liquid mercury.
Finally, also in the laboratory the study of the interaction of cavities inside a uniform fermionic background is of importance (Bulgac and Wirzba., 2001).
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