Rutherford’s discovery aroused intense curiosity about the nature and
possible structure of this extremely small, but all-important, part of
an atom. It was assumed that the positive charge carried by the nucleus
must be a whole-number multiple of a small unit equal in size but
opposite in sign to the charge of an electron. This conclusion was based
on the information that all atoms contain electrons and that an
undisturbed atom is electrically neutral. Since it was known that a
neutral atom of hydrogen contains just one electron, it appeared that
the charge on a hydrogen nucleus must represent the fundamental unit of
positive charge, some multiple of which would represent the charge on
any other nucleus. Several lines of investigation combined to establish
quite firmly that nuclei of atoms occupying adjacent positions on the
periodic chart of the elements differed in charge by this fundamental
unit. Since the hydrogen nucleus seemed to play such an important role
in making up the charges of all other nuclei, it was given the name
proton from the Greek “protos,” which means “first.”