Scientists concluded that most of an atom is empty space through Ernest Rutherford's gold foil experiment in 1909. In this experiment, alpha particles (positively charged and relatively massive) were directed at a very thin sheet of gold foil. Most alpha particles passed straight through the foil without deflection, indicating that the atom is mostly empty space. However, a small number of alpha particles were deflected at large angles or bounced back, suggesting the existence of a tiny, dense, positively charged nucleus at the center of the atom where most of the mass is concentrated. This was surprising because the then-accepted plum pudding model predicted alpha particles would mostly pass through with minimal or no deflection. Rutherford concluded that the atom's positive charge and most of its mass reside in this small nucleus, surrounded by mostly empty space where electrons are located. The nucleus is about 1/10,000th the size of the atom, which explained why most alpha particles were not deflected as they passed through the vast empty space around it.
