In the early 20th Century the plum pudding model of the atom was replaced by the nuclear model of the atom, explain why this happened.

The plum pudding model defined the atom as a particle in which the charge and the mass of the atom was spread evenly. An experiment was set up in which alpha particles where shot to a thin gold foil. The expected outcome by this model was that the alpha particles would be evenly scattered by the atoms of the gold foil. However, the result was that most of the alpha particles went through the gold foil very easily, meaning that there was a lot of space between atoms. Thus, this would eventually lead to the creation of the nuclear model of the atom, in which the atom is defined as having the charge spread around it (in the form of electrons orbiting the nucleus) and all of the mass concentrated in the center of the atom, with a very large distance (in atomic scale) between the mass and the charge.