The charge cloud model is an atomic model postulated by Erwin Schrödinger that takes into account the particle-wave duality of electrons orbiting an atomic nucleus. This model assigns a cloud of probability densities in regions around an atomic nucleus where electrons can be found with a high degree of certainty.
The electron cloud around the atomic nucleus represents a history of where the electron has probably been before and where it is likely to go. As the electron moves from one location to another around the atomic nucleus, it can be thought to leave an imprint of where it was. The superposition of these imprints together forms the electron cloud.
Regions where the cloud is most dense represent regions that have the highest likelihood of having an electron present. Conversely, fuzzy and undefined regions of the cloud represent regions where the likelihood of finding the electron is low. Regions in which there is no cloud represent places where it is impossible to find an orbiting electron. This model was the first to introduce the concept of sub-energy levels. Before the model was introduced, only the primary energy levels defined by the principle quantum numbers in the Bohr atomic model were used to describe where an electron was around the nucleus.