How did quantum mechanics change our understanding of atoms?

It has changed our basic understanding about stability and location of an electron in an atom.

It has changed the way we look at an atom

If we look at the shell model we get a very 2d view , the make us realize that the location of an electron is not limited upto their orbits, but rather there are only 90% chances of electrons to be detected in orbits. The big change in understanding that happened with Quantum Physics is the idea that the universe is random, rather than clockwork, at its lowest level.