Ernest Rutherford's experiment led him to change the outlook on atom because

What led him to change his outlook was because in his experiment, when he fired positively charged alpha particles at a piece of gold foil, he observed that some of the alpha particles were deflected to some degree, and some others bounced back directly toward him.

Explanation:

In his experiment, he fired positively charged alpha particles at a piece of gold foil.

Now, after doing that he observed that some of the alpha particles were deflected to some degree, and some others bounced back directly toward him.

After those observations, he felt that for the order of the alpha particles to have been deflected, they would possibly have been hit or come close to being hit by a positively charged particle in the atom. This made him to conclude that an atom consists mostly of empty space, with a very small and dense but positively charged nucleus at the center. And that this nucleus contains most of the mass of the atom, while the electrons orbit the nucleus.