what is this particle mentioned above?
Subatomic particles are smaller than atoms. They can be composite particles, such as the neutron and proton; or elementary particles, which according to the standard model are not made of other particles.
Subatomic particle
In the physical sciences, subatomic particles are smaller than atoms. They can be composite particles, such as the neutron and proton; or elementary particles, which according to the standard model are not made of other particles. 
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 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. who discovered that it has two charges? French chemist Charles François du Fay
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Subatomic particle [SEP] Introduction
In the physical sciences, subatomic particles are smaller than atoms. They can be composite particles, such as the neutron and proton; or elementary particles, which according to the standard model are not made of other particles. Particle physics and nuclear physics study these particles and how they interact. The concept of a subatomic particle was refined when experiments showed that light could behave like a stream of particles (called photons) as well as exhibiting wave-like properties. This led to the concept of wave–particle duality to reflect that quantum-scale behave like both particles and waves (they are sometimes described as wavicles to reflect this).