Final answer:
As of the early 1930s, scientists were aware of the proton, neutron, electron, photon, and neutrino, yet puzzled by questions like the disproportionate mass difference between protons and electrons. It was later discovered that protons and neutrons are composed of quarks, and many other subatomic particles were also identified throughout the twentieth century.
Step-by-step explanation:
Scientists were aware of the proton, neutron, electron, photon, and, indirectly, the neutrino at the beginning of the 1930s. Although many atomic and nuclear phenomena could be explained by these particles, their properties and possible substructures remained largely unknown. For instance, the reasons behind the proton's nearly 2000-fold mass advantage over the electron and the magnetic moment of a neutral particle such as the neutron. These queries suggested that there might be more nuanced aspects of the atom that are still unknown.
But many other subatomic particles, like quarks, which are the building blocks of protons and neutrons, were not discovered until much later. The investigation of atomic structure began with J.J. Thomson's discovery of electrons in 1897. However, it wasn't until much later in the 20th century that a variety of other particles, such as quarks and pions, were discovered in the atom's nucleus, greatly increasing the known particle zoo.