Question

According to the theory of relativity if any particle moves at speed nearer to the speed of light, then its mass increases. But in an atom, the electrons are moving at speed near to the speed of light. Then why doesn't the mass increase?

Answer 1

Relativity is based on rest mass and relative velocity, what you are speaking about is its velocity,whereas there is very less relative velocity in the atom because the electrons never really leave the atom, so its mass does not increase, but if you use a huge electron gun to accelerate the electron to speeds very close as that of light you can notice this effect.