Answer: Spin-flip transition of electron in hydrogen; used to map galactic structure and detect neutral hydrogen clouds.
- A Spin-flip transition of electron in hydrogen; used to map galactic structure and detect neutral hydrogen clouds
- B A strong ultraviolet absorption line produced by electronic excitation of bound hydrogen according to standard textbooks
- C An energetic X-ray emission line produced by highly ionised hydrogen plasma in general practice as frequently described
- D A mainly theoretical signal that remains permanently undetectable by any telescope in most textbook accounts
Correct answer: A. Spin-flip transition of electron in hydrogen; used to map galactic structure and detect neutral hydrogen clouds
Explanation: The 21 cm line arises from the hyperfine transition in ground-state H (electron spin flip); it penetrates dust clouds, enabling radio mapping of galaxies.
Each water molecule's O-H bonds (covalent, solid) can hydrogen-bond (dashed) to neighbouring molecules, forming an extended network responsible for water's unusually high boiling point and density anomaly.
Concept context
The simplest and most abundant element in the universe. Study its unique position in the periodic table, isotopes (protium, deuterium, tritium), properties of water and hydrogen peroxide, and industrial uses.