Why atomic spectra measures cations only?
Much of what falls into the category of spectroscopy involves wavelengths in or near to the visible part of the spectrum. The methods further involve the absorption of some wavelengths of light leaving others to pass through to your eye or to a detector. Spectroscopy usually involves investigating the wavelengths that are absorbed. Because the wavelengths of light absorbed by d-block electrons in transition metals fall into the visible range, it is these metals which can be investigated with spectroscopy. On the other hand, many monatomic anions have filled d-sublevels and do not absorb at wavelengths near the visible part of the spectrum. That said, iodide ions are colored and can be analyzed by spectroscopic means. Take for example NaCl, CuCl2 and ZnCl2. By Na+ and Zn2+ do not absorb energy in the visible part of the spectrum and are colorless. As a result they are not studied spectroscopically. Na has no d-electrons and the d-sublevel for Zn is filled, therefore, there can be no spl
