4: The problem with classical mechanics
So far we have contrasted classical and quantum motion, but we haven't discussed why classical mechanics breaks down at the molecular level. Here we give one of the reasons why scientists began to doubt the validity of classical mechanics for atoms and molecules.
Classical physics predicts some rather dire (and luckily unobserved!) behavior in atoms. Classical mechanics predicts circular orbits of an electron around a proton like the orbits of the planets around the sun. However, classical electromagnetism predicts that a time-varying electrical field gives rise to electromagnetic radiation. Put simply, a hydrogen atom, according to classical physics, acts like a miniature radio station, as shown in the figure
A circular orbit for an electron orbiting a proton is shown as a heavy black circle in the upper left figure. However, according to the laws of classical physics, the atom will emit electromagnetic radiation. Therefore, the electron will lose energy in the form of radiation and eventually tumble into the proton. The trajectory followed by an electron spiraling into the proton is shown with a dashed line in the upper left. Therefore, classical physics predicts that atoms as we know them are unstable! In quantum mechanics this problem does not arise because electron waves are spread out over a region of space. The uncertainty principle makes it impossible for the electron to collapse into the proton.