The valence shell of an atom is the outermost shell of the electron cloud. It plays a large part in determining the chemical, thermal, optical and electrical properties of the element. This occurs because it often not full and movement of electrons may occur between it and a) electrons from other shells; b) free electrons; c) electrons from other elements.
A valence shell may not have any more than eight electrons. Metals tend to have between 1 and 3 valence electrons and one of these is easily detached from the atom and is described as being 'free'. This accounts for their good electrical and thermal conductivity.
The properties of X-rays and their interaction with matter has a great deal to do with the orbiting electrons, particularly those in the inner shells.