Properties of superfluid Helium

[gendou2]

h ttp://en.wikipedia.org/wiki/Superfluid

Wikipedia states that, in the context of superfluids:

Helium-4 atoms are bosons [whereas] helium-3 atoms are fermions.

I assume what is meant is that the atoms have bosonic an fermionic properties under super-cooled conditions.
I gather that the spin of the constituents of the atom (protons, neutrons, electrons) are added up to predict the properties.
My question is, why does the atom behave like one quantum mechanical entity, having it's own spin, when it is actually many parts?

I'm sorry for asking a sophomoric question.

 

[Gokul43201]

It depends on the nature of the interaction that is being studied. Just as a comet orbiting the sun can be treated as essentially a point object (containing the mass of all its atoms added together) for the purpose of describing its gravitational interaction with the Sun, atoms can be treated as composite fermions or bosons (adding the spins of its components) for the purpose of studying interactions in say, a superfluid system.

It is thus important to keep track of the length scales involves in the problem. For example, even in liquid helium, the typical interatomic separation is at least an order of magnitude bigger than the atomic radius and several orders of magnitude bigger than the nuclear radius.

 

[gendou2]

Fascinating. Thanks for the great explanation.