Why don't electrons crash into the nucleus?

Electrons don't crash into the nucleus due to the balance between their kinetic energy and electrostatic attraction.

In more detail, the behaviour of electrons in an atom is governed by the principles of quantum mechanics. According to the Heisenberg Uncertainty Principle, it is impossible to simultaneously know both the exact position and momentum of a particle. This means that an electron in an atom cannot be at a definite place at a definite time. Instead, it exists in a cloud-like region around the nucleus known as an electron cloud or orbital, where the electron's position is described by a probability distribution rather than a precise location.

The electrostatic attraction between the positively charged nucleus and the negatively charged electron tends to pull the electron towards the nucleus. However, the electron also possesses kinetic energy due to its motion. If the electron were to move too close to the nucleus, its speed (and hence its kinetic energy) would have to increase significantly to comply with the Uncertainty Principle. This increase in kinetic energy would counteract the electrostatic attraction, pushing the electron away from the nucleus and preventing it from crashing into it.

Furthermore, the concept of energy levels or shells also plays a crucial role. Electrons occupy specific energy levels, and they can only 'jump' from one level to another by absorbing or emitting energy. The energy levels closest to the nucleus are the lowest energy states that electrons can occupy, and they are typically filled first. Once these levels are filled, additional electrons occupy higher energy levels further from the nucleus. This arrangement of electrons in energy levels further prevents them from simply falling into the nucleus.

In essence, the principles of quantum mechanics, particularly the Heisenberg Uncertainty Principle and the concept of energy levels, ensure that electrons maintain a certain average distance from the nucleus, preventing them from crashing into it.