which theory explains or is based on this?
Atomic Theory, Quantum Theory
Electron
Quantum Theory
where are electrons found in an atom and what electrical charge and mass do they have An electron can be "bound" to the nucleus of an atom by the attractive Coulomb force.  If the number of electrons is different from the nucleus' electrical charge, such an atom is called an ion.
wiki:1606636
Electron [SEP] Quantum mechanics
In his 1924 dissertation "" (Research on Quantum Theory), French physicist Louis de Broglie hypothesized that all matter can be represented as a de Broglie wave in the manner of light. That is, under the appropriate conditions, electrons and other matter would show properties of either particles or waves. The corpuscular properties of a particle are demonstrated when it is shown to have a localized position in space along its trajectory at any given moment. The wave-like nature of light is displayed, for example, when a beam of light is passed through parallel slits thereby creating interference patterns. In 1927, George Paget Thomson discovered the interference effect was produced when a beam of electrons was passed through thin metal foils and by American physicists Clinton Davisson and Lester Germer by the reflection of electrons from a crystal of nickel.