Extending human observations of natural phenomena further, the great minds of 20th century studied the nature and behavior of matter and energy at a very small scale. It was Rutherford who proposed that mass of an atom was concentrated in the nucleus and that the nucleus has a positive charge. He also added that the nucleus was surrounded by negatively charged electrons. Extending this model, Neils Bohr placed the electrons in a definite shell, or quantum shells. These observations led to new insights into the arrangement of electrons around nucleus and the processes by which the arrangements change. The field of atomic theory was born.
The discovery of neutron by James Chadwick, on the other hand, led to a new field called nuclear theory. This theory proposed a new type of force called nuclear force. Physicists soon had four fundamental forces in nature to contend with: strong nuclear force, weak nuclear force, gravitation, and electromagnetism. Now, the attention of physicists turned deeper into the structure of nucleons, it was Murray Gell-Mann who proposed that nucleons were composed of quarks. The deeper physicists travelled, they observed that a different picture of space, time, and matter presented itself (as compared to the classical physics). These phenomena are now well explained by advancements, in what is termed, the modern physics.