Nearly all the mass of an atom resides in the nucleus.
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The positively charged center in an atom is called the nucleus.
Thomson likened the atom to a Christmas pudding or a watermelon, where electrons were compared to currants or seeds embedded within a positively charged sphere.
It revolutionized the understanding of atomic structure, revealing the nucleus as a tiny, dense center of positive charge surrounded by mostly empty space occupied by electrons, leading to the development of the nuclear model of the atom.
Rutherford calculated that the radius of the nucleus is about 10^5 times less than the radius of the atom based on the observations of the deflected α-particles.