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According to the passage, in the early twentieth century, physicists ----.
In the early part of the twentieth century, the
experiments carried out by Ernest Rutherford and his
colleagues led to the idea that at the centre of an
atom there is a tiny but massive nucleus. At the same
time that the quantum theory was being developed
and that scientists were attempting to understand the
structure of the atom and its electrons, investigations
into the nucleus itself had also begun. An important
question to physicists was whether the nucleus had a
structure, and what that structure might be. In fact, it
has so far turned out that the nucleus is a
complicated entity, and even today, it is not fully
understood. However, by the early 1930s, a model of
the nucleus had been developed that is still useful.
According to this model, a nucleus is considered as
an aggregate of two types of particles: protons and
neutrons. A proton is the nucleus of the simplest
atom which is hydrogen. The neutron, whose
existence was ascertained only in 1932 by the
English physicist James Chadwick, is electrically
neutral as its name implies. These two constituents of
a nucleus, neutrons and protons, are referred to
collectively as “nucleons.”