Why do atoms give a line spectrum?

The bottom line is that absolutely all substances in a gaseous + atomic (“indivisible”) state emit the aforementioned line spectrum. Line spectra represent ordinary colored lines with separated intervals. If you go deeper into the question, then the spectrum is inherent in any atom and each atom has its own. The spectrum is a kind of distinctive and authentic characteristic of one atom that completely distinguishes it from other atoms. Each electron transfer has its own wavelength.