Re: Proton probability cloud

This question has answers at several different levels. The first, and 
simplest answer runs as follows:

If we are thinking of a stationary, isolated atom of hydrogen-1, there
are only two particles present - a proton and an electron. The proton is
about 2000 times as massive as the electron. We normally think of the proton 
as infinitely massive by comparison with the electron. But if we want to be 
more realistic, we must remember that both proton and electron will move.
But the centre-of-mass of the two particle system will remain stationary.
Whenever the electron moves anywhere, the proton will move 1/2000 of the 
distance in the opposite direction. This behaviour in classical mechanics
continues much the same way in quantum mechanics. Motions of the electron
and proton are exactly correlated. The electron and proton ***share*** the
same wavefunction. If it is the 1s wavefunction, then at the same time as 
it spreads the electron over a sphere of about 600 picometres radius, it
will spread the proton over a corresponding sphere 2000 times smaller -
about 300 femtometres radius.

There are two other senses in which we think about the wavefunctions of an
atomic nucleus, or of the particles in an atomic nucleus. The first is the
concern of particle physicists, who think about the way the protons and
neutrons move around past one another within the region of a multi-particle
atomic nucleus. In this sense, a proton in a multi-particle nucleus can
have its own wavefunction which is very similar to that of an electron in 
an atomic orbital, but on a much smaller scale. The second sense is used
mainly by chemists, who need to consider the motions of the two atoms at
the ends of a chemical bond in a molecule, and how they vibrate, in a 
quantum mechanical sense. These vibrational wavefunctions are usually
ascribed to the atomic nucleus as a whole, and are usually much more
complicated in shape than electron wavefunctions because the geometry of
any molecule is much less symmetric than the spherical geometry of an atom.

John.