According to Thomson model, the mass of the atom is assumed to be uniformly distributed over the atom.

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Check out Thomson's model.

In 1898, J.J. Thomson proposed that an atom possesses a spherical shape (radius approximately ${10}^{-10}m$) in which the positive charge is uniformly distributed. Electrons are embedded into it more or less uniformly distributed to give the most stable electrostatic arrangement. This model of the atom is known as plum pudding model or raisin pudding model or watermelon model. This model can be visualized as pudding or watermelon of positive charges with plums or seeds (electrons) embedded into it.

This model assumed that the mass of an atom is uniformly distributed over the atom.