In a hydrogen atom the negatively charged electron is circling around the positively charged proton. Unlike charges are strongly attracted to each other, so why doesn't the electron fall toward the proton?

Gnomon

The traditional view was that the electron was orbiting around the nucleus and that it was forced into a circular orbit by the attractive force of the positive nucleus, in the same way that the earth is forced into a circular orbit around the sun. The modern quantum mechanical view is that the electron does not exist at any one point, but is smeared out over an area called the quantum mechanical shell. Since the electron exists at all points at once, it can not fall into the centre. Dec 16 02, 11:20 PM

Bryce

It turns out that there is a certain distance between the electron and the proton at which the electron feels the most comfortable. For that distance, called the Bohr radius, the energy of the proton-and-electron system reaches a minimum, and the system becomes stable. Dec 17 02, 12:02 AM