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J. J. Thomson's contribution to the understanding of the atom's structure includes the discovery of electrons in 1897 and proposing the "plum pudding model," where negatively charged electrons are dispersed throughout a positively charged "soup." This discovery played a significant role in understanding the nature and behavior of atoms. Ernest Rutherford, through his gold foil experiment in 1909, discovered the nucleus of an atom, which he deduced was small, dense, and positively charged. Rutherford's discovery led to the modern understanding of atomic structure, known as the Rutherford model or the nuclear model of the atom, where the majority of an atom's mass is concentrated in its nucleus, with electrons orbiting around it.