The second statement describes Rutherford’s explanation of the alpha scatter experiment (”gold foil experiment”) conducted by Hans Geiger and Ernest Marsden. Rutherford figured that since most of the alpha particles traveled straight through the gold and were undeflected, that gold was mostly empty space, and that almost the entire mass of the gold atom was found in the nucleus

The alpha particles, with their positive electrical charge, were repelled by the positive charge on the nuclei of atoms.

For your last statement, an atom of Mo-90 contains 42 protons. Subtract 42 from 90, to get the number neutrons in the nucleus. There are 48 neutrons in an atom of Mo-90

Molybdenum is #42, showing that it has 42 protons. In molybdenum-90, 90 is the mass (mass of electrons are ignored). So 90-42 = 48 nuetrons.