(1) Isotopes of a certain element are identical in their chemical properties-why?
(2) Atomic spectra of elements are called finger-prints-why?

Explanation

(1) isotopes of an element have same number of protons in their nucleus and same number electrons in the extranuclear part. Thus, they have same electronic configuration in their valence shell and hence identical chemical properties.
(2) The atomic spectra (line spectra) of each element is unique. the line spectra of no two elements resemble each other. hence, these are regarded as the finger prints of the elements and thus helps in their identification.