<P> Electrons are identical particles because they cannot be distinguished from each other by their intrinsic physical properties . In quantum mechanics, this means that a pair of interacting electrons must be able to swap positions without an observable change to the state of the system . The wave function of fermions, including electrons, is antisymmetric, meaning that it changes sign when two electrons are swapped; that is, ψ (r, r) = − ψ (r, r), where the variables r and r correspond to the first and second electrons, respectively . Since the absolute value is not changed by a sign swap, this corresponds to equal probabilities . Bosons, such as the photon, have symmetric wave functions instead . </P> <P> In the case of antisymmetry, solutions of the wave equation for interacting electrons result in a zero probability that each pair will occupy the same location or state . This is responsible for the Pauli exclusion principle, which precludes any two electrons from occupying the same quantum state . This principle explains many of the properties of electrons . For example, it causes groups of bound electrons to occupy different orbitals in an atom, rather than all overlapping each other in the same orbit . </P> <P> In a simplified picture, every photon spends some time as a combination of a virtual electron plus its antiparticle, the virtual positron, which rapidly annihilate each other shortly thereafter . The combination of the energy variation needed to create these particles, and the time during which they exist, fall under the threshold of detectability expressed by the Heisenberg uncertainty relation, ΔE Δt ≥ ħ . In effect, the energy needed to create these virtual particles, ΔE, can be "borrowed" from the vacuum for a period of time, Δt, so that their product is no more than the reduced Planck constant, ħ ≈ 6984660000000000000 ♠ 6.6 × 10 eV s . Thus, for a virtual electron, Δt is at most 6979129999999999999 ♠ 1.3 × 10 s . </P> <P> While an electron--positron virtual pair is in existence, the coulomb force from the ambient electric field surrounding an electron causes a created positron to be attracted to the original electron, while a created electron experiences a repulsion . This causes what is called vacuum polarization . In effect, the vacuum behaves like a medium having a dielectric permittivity more than unity . Thus the effective charge of an electron is actually smaller than its true value, and the charge decreases with increasing distance from the electron . This polarization was confirmed experimentally in 1997 using the Japanese TRISTAN particle accelerator . Virtual particles cause a comparable shielding effect for the mass of the electron . </P>

Where are electrons found in an atom & describe their movement