Question: What rule is based on observation of atomic spectra?

Context paragraphs:
Walther Kossel: Walther Ludwig Julius Kossel (4 January 1888 in Berlin, Germany – 22 May 1956 in Tübingen, Germany) was a German physicist known for his theory of the chemical bond (ionic bond/octet rule), Sommerfeld–Kossel displacement law of atomic spectra, the Kossel-Stranski model for crystal growth, and the Kossel effect.  Walther was the son of Albrecht Kossel who won the Nobel Prize in Physiology or Medicine in 1910.

Sudarsky's gas giant classification: Sudarsky's classification of gas giants for the purpose of predicting their appearance based on their temperature was outlined by David Sudarsky and colleagues in the paper "Albedo and Reflection Spectra of Extrasolar Giant Planets" and expanded on in "Theoretical Spectra and Atmospheres of Extrasolar Giant Planets", published before any successful direct or indirect observation of an extrasolar planet atmosphere was made.  It is a broad classification system with the goal of bringing some order to the likely rich variety of extrasolar gas-giant atmospheres.

Friedrich Hund: Friedrich Hermann Hund (4 February 1896 – 31 March 1997) was a German physicist from Karlsruhe known for his work on atoms and molecules.

Charge radius: The rms charge radius is a measure of the size of an atomic nucleus, particularly of a proton or a deuteron.  It can be measured by the scattering of electrons by the nucleus and also inferred from the effects of finite nuclear size on electron energy levels as measured in atomic spectra.

Heinrich Gerhard Kuhn: Heinrich Gerhard Kuhn {'1': ", '2': ", '3': ", '4': "} (10 March 1904 – 25 August 1994) was a British physicist.  A graduate of the University of Göttingen, where he studied for his doctorate under the direction of James Franck, winner of the 1925 Nobel Prize for Physics, he left Germany after the Nazi Party came to power there in 1933, and moved to Britain, where relatives had settled, becoming a British subject in 1939.  At the invitation of Frederick Alexander Lindemann, he worked for Imperial Chemical Industries at the Clarendon Laboratory in Oxford, where he studied hyperfine structure.  During the Second World War, he worked on isotope separation for Tube Alloys and the Manhattan Project.  He was the first physicist to become a fellow at Balliol College, Oxford, in 1950, and published textbooks on atomic spectra in German in 1934 and English in 1962.

Atomic spectroscopy: Atomic spectroscopy is the study of the electromagnetic radiation absorbed and emitted by atoms.  Since unique elements have characteristic (signature) spectra, atomic spectroscopy, specifically the electromagnetic spectrum or mass spectrum, is applied for determination of elemental compositions.  It can be divided by atomization source or by the type of spectroscopy used.  In the latter case, the main division is between optical and mass spectrometry.  Mass spectrometry generally gives significantly better analytical performance, but is also significantly more complex.  This complexity translates into higher purchase costs, higher operational costs, more operator training, and a greater number of components that can potentially fail.  Because optical spectroscopy is often less expensive and has performance adequate for many tasks, it is far more common Atomic absorption spectrometers are one of the most commonly sold and used analytical devices.

Hund's rule of maximum multiplicity: Hund's rule of maximum multiplicity is a rule based on observation of atomic spectra, which is used to predict the ground state of an atom or molecule with one or more open electronic shells.  The rule states that for a given electron configuration, the lowest energy term is the one with the greatest value of spin multiplicity.  This implies that if two or more orbitals of equal energy are available, electrons will occupy them singly before filling them in pairs.  The rule, discovered by Friedrich Hund in 1925, is of important use in atomic chemistry, spectroscopy, and quantum chemistry, and is often abbreviated to Hund's rule, ignoring Hund's other two rules.

Rydberg constant: The Rydberg constant, symbol "R" for heavy atoms or "R" for hydrogen, named after the Swedish physicist Johannes Rydberg, is a physical constant relating to atomic spectra, in the science of spectroscopy.  The constant first arose as an empirical fitting parameter in the Rydberg formula for the hydrogen spectral series, but Niels Bohr later showed that its value could be calculated from more fundamental constants, explaining the relationship via his "Bohr model".  As of 2012, "R" and electron spin g-factor are the most accurately measured fundamental physical constants.

Spectra Records: The Spectra Music Group is a United States music company founded in February 1997 comprising the independent record companies Spectra Records, Monarchy Records, Spectra Jazz and Spectra Heritage—which is based in the United States, Canada and United Kingdom.

Isomeric shift: The isomeric shift (also called isomer shift) is the shift on atomic spectral lines and gamma spectral lines, which occurs as a consequence of replacement of one nuclear isomer by another.  It is usually called isomeric shift on atomic spectral lines and Mössbauer isomeric shift respectively.  If the spectra have also hyperfine structure the shift refers to the center of gravity of the spectra.  The isomeric shift provides important information about the nuclear structure and the physical, chemical or biological environment of atoms.

Supporting paragraphs: Hund's rule of maximum multiplicity Friedrich Hund
Answer: Friedrich Hund