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As a high school student, I'm curious whether everything emits thermal radiation, including monatomic gases and substances composed of a single type of atom and no molecules. Can these objects emit radiation in all wavelengths, and if so, how is it possible in the absence of molecules?Is it necessary for an object to be a blackbody in order to emit thermal radiation across all wavelengths?

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Isolated atoms can emit and absorb only specific discrete wavelengths. A monatomic gas at low density approximates to a collection of isolated atoms. But there are at least three reasons why the equivalence is not exact...

(a) The atoms are constantly colliding so are not isolated. This results in broadening of the absorption and emission 'lines'.

(b) Because the atoms are in constant random motion relative to any observer's spectroscope, there will be line broadening due to the Döppler effect.

(c) In practice the gas will be contained in a container made of solid material. This will absorb and emit a virtually continuous spectrum of wavelengths owing to the close spacing of its energy levels.

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  • $\begingroup$ While your (c) is definitely sufficient, your (a) and (b) are simply never going to produce the necessary blackbody radiation curve. At most you can have some broadening over where the discrete spectrum would be, and nowhere else. (c) is also wrong because there can be band gaps. $\endgroup$ May 9, 2023 at 5:13
  • $\begingroup$ @naturally inconsistent Did I claim that these factors would produce a black body spectrum? $\endgroup$ May 9, 2023 at 7:21
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From Basics of Radio Astronomy, published by NASA's Jet Propulsion Laboratory in April 1998, Chapter 3 - The Mechanisms of Electromagnetic Emissions (emphasis added):

In solids, the molecules and atoms are vibrating continuously. In a gas, the molecules are really zooming around, continuously bumping into each other. Whatever the amount of molecular motion occurring in matter, the speed is related to the temperature. The hotter the material, the faster its molecules are vibrating or moving.

Electromagnetic radiation is produced whenever electric charges accelerate—that is, when they change either the speed or direction of their movement. In a hot object, the molecules are continuously vibrating (if a solid) or bumping into each other (if a liquid or gas), sending each other off in different directions and at different speeds. Each of these collisions produces electromagnetic radiation at frequencies all across the electromagnetic spectrum. However, the amount of radiation emitted at each frequency (or frequency band) depends on the temperature of the material producing the radiation.

[...]

Any matter that is heated above absolute zero generates electromagnetic energy. The intensity of the emission and the distribution of frequencies on the electromagnetic spectrum depend upon the temperature of the emitting matter. [...]

The author provides a table on page 3 of the PDF (page 21 in the book):

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Thus, according to the JPL, monatomic gases do indeed emit radiation in all wavelengths (according to their temperature), just like liquids and solids do.

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