Atomic Physics - Bohr's model of atom Well I'm learning about the models that have been proposed for the atom, and the Bohr model came up. 
My teacher told me that the one of the main postulate of the theory is that when an atom is in ground state, it can neither absorb nor emit energy. This is what gives the atom its stability. However, it can obviously absorb energy to jump to a higher energy level. I don't understand, how can it jump to a higher energy level, from the ground state obviously, without absorbing energy? Some problem here?
 A: In Bohr`s model postulates are
1.Electrons in atoms orbit the nucleus.
2.The electrons can only orbit stably, without radiating, in certain orbits (called by Bohr the "stationary orbits") at a certain discrete set of distances from the nucleus. These orbits are associated with definite energies and are also called energy shells or energy levels. In these orbits, the electron's acceleration does not result in radiation and energy loss as required by classical electromagnetics.(these postulates states atomic stability)
3.Electrons can only gain and lose energy by jumping from one allowed orbit to another, absorbing or emitting electromagnetic radiation with a frequency ν determined by the energy difference of the levels.
so it does requires energy to be absorbed to make transition from lower energy level to higher energy level. And it will radiate energy while coming to lower energy level from higher energy level.
you can read everything about bohr`s model on Wikipedia link here: http://en.wikipedia.org/wiki/Bohr_model
Hope this will help you
