The spectrum of an excited gas is a series of discrete lines because

The spectrum of an excited gas is a series of discrete lines because of electronic transition due to radiation coming from a source that it is not in the same field of view of the gas with respect to the observer.

Explanation:

Kirchhoff’s laws establish that:

   • A solid, liquid or dense incandescent gas emits a continuous spectrum.  

   • A hot and diffuse gas produces bright spectral lines (emission lines).

   • A gas of lower temperature against a source of continuum spectrum, produces dark spectral lines (absorption lines) superposed in the continuum spectrum.

The atomic emission spectrum of an element has its origin when an electronic transition¹ occurs. An electron in an atom or ion² will absorb energy coming from a source and pass to a higher energy level, the electron, upon returning to its base state will emit a photon³ or a series of photons. Hence, that leads to the formation of an emission spectrum since the observer is not seeing the cloud with the luminous source behind (which generates an absorption spectrum).

Remember that an electron has energy levels in an atom or ion, at which each energy level has an specific value.

The energy values will differ from one element to another, so it can be concluded that each element has a unique pattern of emission lines.

Key terms:

¹Electronic transition: When an electron passes from one energy level to another, either for the emission or absorption of a photon.

²Ion: An atom electrically charged due to the gain or loss of electrons.  

³Photon: Elementary particle that constitutes light.

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