I have read this in my book:
In solids, very large numbers of atoms are squeezed into a small volume and there is considerable interaction between them. As a result, all the electrons traveling in the first orbits will have slightly different energy levels because no two of them will see exactly the same charge environment.

Can anyone explain simply? Why interaction between them make the different enery levels?

 

Every charge has a field of effect around them, called the electric field. Now, in an atom, the protons and each of the electrons contribute to this field. This field is dependent on distance, it's a vector, so also on direction. Thus, if electron A is under the influence of an electric field described by a vector (which is just something used to show the direction and magnitude of a quantity), electron B will have a different vector for electric field acting on it. This is because it is in a different position relative to the other electron. In solids, there are lots of electrons and protons contributing to the field. As the position of a charge varies, so does the electric field acting on it.

Thus, no two electron will have exactly the same electric field(charge environment, as your book calls it).

 

Hello,

Have a look at the wiki article:
http://en.wikipedia.org/wiki/Energy_bands

Bertus