Electron Affinity 
 When an electron is removed from an orbital, energy is needed, and therefore work has to be done.  Each electron in an atom is attracted to the nucleus, and this attraction must be overcome if an electron is to be removed.  If an extra electron is added to an atom to create a negative ion, this electron will also come under the influence of the nucleus, and work will have to be done to remove it to make the atom neutral once again. The energy needed to remove an electron from a negative ion is equal in magnitude to the energy given off when the electron is added to the atom to create the ion. This energy is called the electron affinity.

                                           X  +  e-   ----->  X-   +  energy

The trends in electron affinity are very similar to those of ionization energy. Electron affinity generally increases from the left to right in a period and increases from bottom to top in a group.   A valence shell that loses electrons easily will have little attraction for additional electrons. On the other hand, a valence shell that holds its electrons tightly will also tend to hold an additional electron tightly.