Final answer:
Hydrogen's 21-cm emission line was predicted by Hendrik van de Hulst in 1944 and first detected by Harvard physicists in 1951, enabling the mapping of neutral interstellar gas in the Galaxy. Hydrogen is mostly found in water and organic compounds on Earth, with its spectral lines key to studying its properties.
Step-by-step explanation:
Hydrogen was discovered as an individual element on Earth, but in the context of astronomy and its presence in the universe, a significant milestone was the detection of the 21-cm line of neutral hydrogen in the Galaxy. In 1944, Dutch astronomer Hendrik van de Hulst predicted that hydrogen would produce a strong emission line at a wavelength of 21 centimeters. The first detection of this line was made by Harvard physicists Harold Ewen and Edward Purcell in 1951, using equipment sensitive enough to detect the weak signal. The discovery allowed astronomers to map out neutral interstellar gas throughout our home Galaxy and was soon confirmed by other groups in the Netherlands and Australia.
Despite hydrogen being one of the most abundant elements on our planet, most of it is bound in water molecules and organic compounds, with very little in its elemental form. This is because hydrogen is extremely light, with a molecular weight of 1, causing any excess to diffuse into space over time, leaving only about 0.5 ppm in the Earth's atmosphere. The spectral lines of hydrogen, such as those predicted by the Rydberg formula, are essential for understanding its physical properties and have been observed in various regions, from the ultraviolet to the infrared.