Oscillation of Atom What exactly does it mean when one says 'one atom of Caesium 137 oscillates 9,192,631,770 times'? I do understand the general thing about oscillation but what exactly is the oscillation of atom, what thing in the atom is actually causing the oscillation?
 A: Due to the quantized energy levels of allowed electron orbitals, single atoms can easily absorb energy around certain narrow wavelengths.
A cesium atom has one of these narrow absorption bands at a frequency of 9,192,631,770 Hz.  A cesium clock can produce EM radiation in this region (microwaves) and detect how well the cesium atoms are absorbing it.  
So the atom isn't really oscillating at that rate.  Instead, a microwave transmitter is tuned to near that rate and attached to circuitry that can count the oscillations of the transmission frequency.
A: My short answer comes from reading this page on the web:
https://www.4physics.com/phy_demo/at_clock/at_clock.htm 
For more details, please go to that page.  It is very clear.
The Cesium atom itself is not oscillating.  The oscillation being measured is the frequency of the photon that is emitted by the Cesium valence electron when it jumps from an excited energy state back down to the ground energy state.  This excitation of the electron is created by radiating the cesium atoms with a microwave beam that is oscillating at a frequency that is equal to the Cesium atom's natural hyperfine transition frequency.  The hyperfine transitions are a result of the interaction of the magnetic field of the spinning nucleus of the Cesium atom, and the electric field of the valence electron of the Cesium atom.  These interactions cause the electron to be excited and then return to the base state.  The photon emitted is in the microwave frequency range; that is 9,192,631,770 hertz.  
A: Each orbit is having its fixed energy level.
the orbital electrons of different orbits give and take energy inside the atom during their revolutions, due to interactions of theirelectrical field betweeneach electron and its corresponding proton in the nucleus.During this give and take electric fields between orbital electrons and the protons are disturbed and they emit electromagnetic pulses out side the atom. This loss of energy is recouped from the surronding energy in space; or special arranged energy source. However, due to this electromagnetic oscillations of internal energy of the atom its mass is also bound to vibrate alongwith the em enrgy pulse emmission.
