what exactly is the part of the atom mentioned above, initially?
The electron is a subatomic particle, whose electric charge is negative one elementary charge.
Electron
The electron is a subatomic particle, symbol or , whose electric charge is negative one elementary charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no known components or substructure. 
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. which theory explains or is based on this? Atomic Theory, Quantum Theory
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Electron [SEP] Introduction
The electron is a subatomic particle, symbol or , whose electric charge is negative one elementary charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no known components or substructure. The electron has a mass that is approximately 1/1836 that of the proton. Quantum mechanical properties of the electron include an intrinsic angular momentum (spin) of a half-integer value, expressed in units of the reduced Planck constant, "ħ". Being fermions, no two electrons can occupy the same quantum state, in accordance with the Pauli exclusion principle.