Complex sentence: They discovered that, when an electron collided with a mercury atom, it could lose only a specific quantity (4.9 electron volts) of its kinetic energy, which corresponds to decelerating the electron from a speed of about 1.3 million meters per second to zero.
Split into simpler sentences: They discovered that, when an electron collided with a mercury atom, it could lose only a specific quantity (4.9 electron volts) of its kinetic energy. This energy loss corresponds to decelerating the electron from a speed of about 1.3 million meters per second to zero.