Question

why spectrum of an atomof an element is regarded as it finger print

Answer 1

Electrons can absorb and emit energy only in certain discrete amounts. When an electron absorbs a photon of light with just the right amount of energy, it jumps to a higher energy level. When the electron spontaneously jumps back to a lower energy level, a photon is emitted. Enough electrons jumping between any two given energy levels of a given element will result in a spectral emission or absorption line at a characteristic wavelength.

Spectral lines are used to identify the element and thus, it is called fingerprint of elements.

Different atoms will have the different spectral lines. Just like two different persons have different finger prints.