Johannes Robert Rydberg

Swedish physicist

• Born: November 8, 1854; Halmstad, Sweden
• Died: December 28, 1919; Lund, Sweden

Johannes Robert Rydberg was a Swedish physicist best known for his studies on the spectral analysis of chemicals. He discovered formulas for predicting spectral lines of chemicals and metals before atomic structure was properly understood. The Rydberg constant is named for him.

Also known as: Janne Robert Rydberg

Primary field: Physics

Specialties: Atomic and molecular physics; physical chemistry

Early Life

Johannes Robert Rydberg was born on November 8, 1854, to Sven Rydberg, a merchant who died just four years later, and Maria Anderson. He studied in his hometown of Halmstad for his primary and secondary education, graduating in 1873. That same year, he enrolled at the University of Lund, where he would study and then teach until his death in 1919. He began by studying pure mathematics, earning his bachelor’s degree in 1875 and his doctorate in 1879. Afterward, he became more interested in physics, and in 1882, he was given the post of assistant lecturer in physics at Lund. His career rose steadily until he was awarded a full permanent professorship of physics in 1901. His appointment came after being denied tenure in favor of another physicist who was a personal friend of King Oscar II of Sweden.

In 1882, Rydberg published his first physics paper on the subject of friction. Early in his physics career, he became interested in the subject that would make him famous—the newly developed periodic table of elements. The modern periodic table is based on the design of Russian chemist Dmitry Ivanovich Mendeleyev (1834–1907), who published his version in 1869. As chemistry became more sophisticated, chemists were beginning to determine the difference between elements, which are made of only one kind of atom, and compounds, which are made of two or more elements in different ratios. Mendeleyev’s periodic table grouped the elements by atomic weight, or, as it is now known, number of protons. Protons had yet to be discovered, however, and this grouping was not perfectly understood. Rydberg’s later work would help define the properties of elements and their position on the periodic table.

Life’s Work

Rydberg began his work in physics soon after the discovery that different chemicals emit unique spectral lines when burned by flames or electricity. When a chemical burns, it emits light; when that light passes through a prism, it fractures into a spectrum, or rainbow, and the width of the different colored lines can be measured. The chemical composition of stars can be determined this way because stars are simply burning balls of chemicals. When a star’s light passes through a prism (or its modern equivalent), it produces spectral lines that can be analyzed to discover which chemicals exist in that star and in what quantities. Chemists found that the same chemical will produce the same spectrum every time, and so they knew that the atoms of the chemical must somehow relate to the spectrum of light produced by the chemical. The connection, however, was not understood.

Rydberg began studying the spectra of elements around 1884. He was determined to quantify and decipher the spectral data that had been collected by other chemists and find what patterns may emerge. He hoped that his research would help him piece together a model of atomic structure, and although he died before he was successful, later scientists based atomic models on his work.

His first step toward finding a mathematical model for the spectra was to assign whole numbers to the spectral lines, starting with one for the shortest wavelength. He called it the wave number and defined it as the number of waves per centimeter of any given color. His solid mathematics background made him uniquely qualified to attempt to quantify physical data, and through trial and error, Rydberg developed his equations for predicting spectral lines, presenting them in 1887. (It is unclear whether he was aware of the work of fellow mathematician Johann Jakob Balmer at this time and, if so, to what extent Balmer’s calculations influenced his own.) Rydberg’s conclusions were published by the Royal Swedish Academy of Scientists in 1890.

Rydberg was correct in assuming that there must be a periodic relationship between the forces drawing atoms together and their atomic weights. After all, the periodic table arranges elements in order of increasing weight, and no elements fall halfway between two adjoining elements; they increase in whole numbers when measured correctly. It is now known that the atomic weight is the weight of the protons in an element, that protons only exist as whole units, and that all protons are the same size and weight.

Rydberg’s equations did not yield exact results, only very good approximations. Although they worked fairly well, neither he nor any other chemist or physicist could explain the physical laws behind them. The true internal structure of the atom remained a mystery, but Rydberg’s formulas would eventually prove useful in its discovery by Danish physicist Niels Bohr in 1913.

Rydberg married Lydia E. M. Carlsson in 1886, and the couple had two daughters and one son. He remained at Lund for the rest of his life. After a stroke in 1911, Rydberg had to curtail some of his teaching duties. In 1915, his failing health forced him to quit active teaching altogether.

In 1919, Rydberg was elected a foreign member of London’s Royal Society, although he was never elected to the Royal Swedish Academy of Sciences. He was nominated for a Nobel Prize in 1917 but did not win. He was nominated again for the 1920 award, but the Nobel committee would not grant awards posthumously, and Rydberg died of a brain hemorrhage in Lund on December 28, 1919.

Impact

Rydberg’s equations hinted at the true nature of atoms, later uncovered completely by Niels Bohr. Using the suggestion of physical structure glimpsed from Rydberg’s equations, and with help from the theories of Max Planck and Ernest Rutherford, Bohr presented an atomic model that is still considered valid. He grouped the positively charged protons and neutral neutrons at the center of the atom, with negatively charged electrons moving in defined orbits around the nucleus. The simplest model of an atom resembles a solar system, with the protons and neutrons as the much larger sun at the center and the tiny electrons orbiting like planets. In reality, electrons can move in three dimensions at a set distance from the nucleus, forming orbiting electron clouds, or electron shells, with different energy states. If an atom picks up energy, the electrons can jump to outer shells farther from the nucleus and jump back down again to release the energy. The release of energy caused by an electron jumping down to a closer orbit is what causes the spectrum of light released by burning chemicals.

In 1954, the Lund University Physics Department held the Rydberg Centennial Conference on Atomic Spectroscopy in honor of the hundredth anniversary of Rydberg’s birth. Niels Bohr was among the distinguished speakers.

Bibliography

Martinson, I., and L. J. Curtis. “Janne Rydberg—His Life and Work.” Nuclear Instruments and Methods in Physics Research Section B 235.1 (2005):17–22. Print. A detailed biographical article of Johannes Rydberg and his work in spectral analysis.

Parker, Barry R. Quantum Legacy: The Discovery that Changed the Universe. New York: Prometheus, 2002. Print. Discusses the use of the Rydberg constant and the physicists who built upon Rydberg’s ideas.

Rigden, John S. Hydrogen: The Essential Element. Cambridge: Harvard UP, 2003. 197–210. Print. Includes a chapter devoted to the discovery of the Rydberg constant and its later applications in physics.

Thomsen, Volker. “Atomic Perspectives: The Spectral Lines of Hydrogen.” Spectroscopy 23.11 (2008): 29–32. Print. Traces the evolution of quantum theory from Johann Balmer through Niels Bohr, placing Rydberg’s contributions in historical context. Includes tables, figures, and mathematical equations.