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Experiments into electricity began to lead the way to the next stage of understanding. It has been known for many hundreds of years that objects can be given "electric charge", which can be made to move as "electricity". It was discovered that metals are particularly good at conducting electricity, and, importantly, that the metal itself does not appear to move when this happens i.e. the atoms of the metal stay put. So whatever the charge is must be smaller than the atoms themselves. In 1897 this idea was proven experimentally, and the particle of electricity came to be called the "electron". This was the birth of subatomic particle physics.
The last century has seen this topic grow rapidly, with theoretical and experimental discoveries being incorporated into new technology that enables further discoveries, and so on. It was realized that atoms could not be comprised entirely of electrons, as all electrons have the same type of electric charge which causes them to be forced apart. An atom must therefore also have enough particles with opposite electric charge to the electron in order to balance the electric forces and keep the atom intact. Experiments in 1917-1919 led to the discovery of a particle that satisfied this requirement, which came to be called the "proton". However, even though the electrons and protons together balance the charge of an atom, they do not usually add to the total mass of an atom. A massive but electrically neutral particle was therefore predicted soon after the discovery of the proton, which was subsequently discovered about a decade later in 1932 and dubbed the "neutron".
The properties of all atoms, and so the patterns of the periodic table, can be explained in terms of electrons, protons and neutrons. An atom of one element is distinguished from an atom of another element by the number of protons it has, which in turn defines the number of electrons it has in order to balance the electric charge. The protons exist in a central nucleus along with the neutrons, which together provide an atom with most of its mass as the electrons are much lighter. The electrons themselves exist in specific orbits around the nucleus, and the number of electrons and their orbital structure determine how the atom behaves chemically.
Despite answering questions about the periodic table of elements, the nuclear model of the atom generated its own set of questions. How come the electron and proton have electric charge, but not the neutron? How come the protons and neutrons exist together in the nucleus, despite all of the protons repelling each other electrically? How come neutrons are not seen on their own outside of the nucleus? And on and on.