Protons are made of Quarks. They are potentially divisible.
[1]http://en.wikipedia.org/wiki/Quark
[2]http://hyperphysics.phy-astr.gsu.edu/hbase/particles/proton....
[3]http://physics.knoji.com/can-protons-or-neutrons-be-split/
[1]http://en.wikipedia.org/wiki/Quark
[2]http://hyperphysics.phy-astr.gsu.edu/hbase/particles/proton....
[3]http://physics.knoji.com/can-protons-or-neutrons-be-split/
There is also the issue of baryon number conservation. This is physical law (as far as we know) that says that the number of baryons must be constant. A proton is a baryon and neuron is a baryon but there are lots more. Anti-particles count negative so we can destroy a proton by combining it with an anti-proton, if we would happen to have one. But we cannot outright split the proton into some smaller set of parts as with an atom.
In the same way that the atom was not divisible 100 years ago?
Yes, they did:
http://en.wikipedia.org/wiki/Bohr_model
From 1913.
Not a complete model, as we now know, but nonetheless, scientists did have a model of the atom 100 years ago.