No matter how hard you try you will never be able to grasp just how tiny, how spatially unassuming, is a proton. It is just way too small. A proton is an infinitesimal part of an atom, which is itself of course an insubstantial thing. Protons are so small that a little dib of ink like the dot on this “i” can hold something in the region of 500,000,000,000 of them, or rather more than the number of seconds it takes to make half a million years. So protons are exceedingly microscopic, to say the very least. ~ Bill BrysonThat excerpt was blogged by Althouse without much further comment. The reader is supposed to recall from high school or college chemistry just how small the proton really is -- it is after all just a nuclear particle.
Protons cluster in every atom except for hydrogen where they appear alone. In humans, protons mostly nucleate in groups of eight (as in oxygen) or six (as in carbon) with attendant neutrons, but they also go it alone in hydrogen.
Despite the proton's exceedingly tiny size in hydrogen, it is readily detected when placed in a magnetic field. They can even be spatially located in soft tissue by MRI. So there's a nice trade off. If only all the smallest and hardest to see elements were so easy to detect.
Hydrogen is also giving us a glimpse into the mind as in MRI imaging of the brain.