Rutherford suggested that most of the mass of the atom is located in the nucleus.

Explanation:

1. Rutherford's Gold Foil Experiment:
Rutherford conducted the famous Gold Foil Experiment in 1911, which provided evidence for the existence of the atomic nucleus. In this experiment, a beam of alpha particles was directed at a thin sheet of gold foil. According to Thomson's model, the plum pudding model, the positive charge was thought to be spread evenly throughout the atom. However, Rutherford's experiment revealed unexpected results.

2. Deflection of Alpha Particles:
Rutherford observed that most of the alpha particles passed straight through the gold foil with little deflection, which supported Thomson's model. However, a small fraction of the alpha particles were significantly deflected and even bounced back at large angles. This indicated the presence of a concentrated positive charge.

3. Rutherford's Nuclear Model:
Based on the observations from the Gold Foil Experiment, Rutherford proposed a new atomic model known as the nuclear model. According to this model, the atom consists of a tiny, dense, and positively charged nucleus at the center. The nucleus contains most of the mass of the atom, while the electrons orbit around it in empty space.

4. Explanation of the Observations:
Rutherford explained the deflection and scattering of alpha particles by suggesting that they were encountering a concentrated positive charge in the form of the nucleus. The alpha particles that passed straight through the foil were not encountering the nucleus directly and hence were not deflected significantly.

5. Confirmation of the Nuclear Model:
Rutherford's nuclear model was further supported by other experiments and observations. For example, the discovery of isotopes and the understanding of nuclear reactions reinforced the idea that the nucleus contains most of the mass of the atom.

In conclusion, it was Ernest Rutherford who suggested that most of the mass of the atom is located in the nucleus based on his famous Gold Foil Experiment. This experiment provided evidence for the existence of the atomic nucleus and led to the development of the nuclear model of the atom.