Electrons Getting mass from energy According to relation $E=mc^2$ energy and mass are equivalent. So, when an electron in its orbit gets energy from out, does its mass increase? If not, where is the energy stored?
I am not understanding how electrons store energy in them . Does it change the mass of electrons or they immediately release the energy
 A: This is called the mass deficit. A free electron has a certain mass, and a free proton also has a certain mass. The mass of a hydrogen atom is less than the sum of those masses. The difference in the mass between the hydrogen atom and the sum of the free proton and electron masses is called the mass deficit, and it is equal to the ionization energy ($/c^2$)
The concept of mass deficit applies in general to all bound systems. The binding energy is a reflection of the mass deficit.
The mass deficit does not belong to either the proton or the electron individually, but to the hydrogen atom as a whole, so the question of where the electron stores that energy does not arise because it is not stored in the electron but in the atom as a whole. For a system to store energy internally requires some internal degree of freedom, and an electron has no internal degrees of freedom. A hydrogen atom does, specifically the degree to which the atom is excited which indicates how close the electron is to the proton in terms of atomic orbitals. Each orbital corresponds to an amount of energy that can be contained by the degree of freedom in the atom. If a particle has no internal degrees of freedom then energy can only be stored in KE. 
A: Rest mass of an electron is always the same, it cannot be changed by interaction with outside bodies (at least in standard theories).
When atomic electrons get excited and acquire energy $\Delta E$ from outside, this energy may go to EM energy of the system or to kinetic energy of its material parts, or to some other kind of energy.
According to relativity and EM theory, provided the energy stays near the system(so it isn't radiated or lost from the system in other way), no matter the details of where the energy is in the system, it will always manifest as increased inertial mass $\Delta E/c^2$ of the whole system.
