= Composite statistics of neutral atoms
Exchanging two identical bound atoms exchanges their constituent <fermions>. An atom with mass number $A$ and atomic number $Z$ contains $A$ nucleons and $Z$ electrons, so its exchange sign is $(-1)^{A+Z}$ when its internal state is preserved. An odd count gives a composite <fermion>; an even count gives a composite <boson>. This is equivalently the integer-versus-half-integer total-spin distinction in the <Spin-statistics theorem>. For example, neutral lithium-6 has nine fermionic constituents and is a fermion, while neutral lithium-7 has ten and is a boson. Counting nucleons alone instead classifies the nucleus, not the neutral atom. This effective composite-particle description requires energies and separations where the bound atom remains intact.
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