Showed that most of the mass and positive charge of an atom is concentrated in its center

This statement is referring to the Rutherford's Gold Foil Experiment. Here are the steps of the experiment:

1. Rutherford and his team took a thin gold foil and bombarded it with alpha particles (which are positively charged).
2. They expected the alpha particles to pass straight through the foil, based on the then-accepted "plum pudding" model of the atom. This model suggested that atoms were a diffuse cloud of positive charge with negatively charged electrons scattered throughout.
3. However, while most of the alpha particles did go straight through the foil, some were deflected at large angles, and some even bounced back.
4. Rutherford concluded from this that most of the atom's mass and positive charge must be concentrated in a small area in the center (which he called the nucleus), while the rest of the atom is mostly empty space. This is because only a very dense, positively charged area could cause the positively charged alpha particles to deflect so dramatically.
5. This experiment led to the development of the nuclear model of the atom, where the atom is mostly empty space, with nearly all its mass and positive charge concentrated in a tiny nucleus at the center.