Which of the following provided evidence that most of the mass of the atom, as well as all of the positive charge, is concentrated in a very small core (the nucleus)? 1. the analysis of x-ray wavelengths 2. the existence of elements with noninteger values for atomic weights 3. the deflection of ions in a mass spectrometer 4. the results of the millikan oil-drop experiment 5. the scattering of α particles by a metal foil

5. the scattering of α particles by a metal foil

Explanation:

This is the classical Rutherford's experiment in which he bombarded a thin foil of gold with alpha particles which are positively charged helium nucleus.

He did observed that most of the particles passed through the foil relatively undeflected or if they were deflected it was by a very small angle.

Once in a while the alpha particle rebounded completely. An analogy is the one typically mentioned that it was as if we throw a ball at a piece of paper and it rebounds toward us.

This observations led Rutherford to conclude that the nucleus of the atom is  very small positely charged and  that the  atom is relatively empty  with electrons of very small masses. His model is referred as the Plum Pudding model and later Bohr modified it to the planetary model.