What kind of element has the greatest tendency to attract electrons? why?

The kind of element that has the greatest tendency to attract electrons are inert gases or noble gases, in the right side of the periodic table. It is because the nuclear charge is bigger for the elements in the right side of the periodic table, and inert gases or noble gases' atom is the best to attract electrons. 

Determine the  wavelength of light emitted when an electron  in an li2+ ion  makes a transition from an orbital with n  =  6 to an orbital with n  = 5.