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In spectroscopy, the **Rydberg constant**, symbol for
heavy atoms or for hydrogen, named after the Swedish physicist Johannes Rydberg, is a physical constant relating to the electromagnetic spectra of an atom. 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 via his Bohr model. As of 2018^{[update]}, and electron spin *g*-factor are the most accurately measured physical constants.^{[1]}

The constant is expressed for either hydrogen as , or at the limit of infinite nuclear mass as . In either case, the constant is used to express the limiting value of the highest wavenumber (inverse wavelength) of any photon that can be emitted from an atom, or, alternatively, the wavenumber of the lowest-energy photon capable of ionizing an atom from its ground state. The hydrogen spectral series can be expressed simply in terms of the Rydberg constant for hydrogen and the Rydberg formula.

In atomic physics, **Rydberg unit of energy**, symbol Ry, corresponds to the energy of the photon whose wavenumber is the Rydberg constant, i.e. the ionization energy of the hydrogen atom in a simplified Bohr model.^{[citation needed]}

The CODATA value is^{[2]}

- = 10973731.568160(21) m
^{−1},

where

- is the rest mass of the electron,
- is the elementary charge,
- is the permittivity of free space,
- is the Planck constant, and
- is the speed of light in vacuum.

The Rydberg constant for hydrogen may be calculated from the reduced mass of the electron:

where

- is the mass of the electron,
- is the mass of the nucleus (a proton).

^{[3]}^{[4]}

^{[5]}

- .

The angular wavelength is

- .

The Bohr model explains the atomic spectrum of hydrogen (see hydrogen spectral series) as well as various other atoms and ions. It is not perfectly accurate, but is a remarkably good approximation in many cases, and historically played an important role in the development of quantum mechanics. The Bohr model posits that electrons revolve around the atomic nucleus in a manner analogous to planets revolving around the sun.

In the simplest version of the Bohr model, the mass of the atomic nucleus is considered to be infinite compared to the mass of the electron,^{[6]} so that the center of mass of the system, the barycenter, lies at the center of the nucleus. This infinite mass approximation is what is alluded to with the subscript. The Bohr model then predicts that the wavelengths of hydrogen atomic transitions are (see Rydberg formula):

where *n*_{1} and *n*_{2} are any two different positive integers (1, 2, 3, ...), and is the wavelength (in vacuum) of the emitted or absorbed light.

where and *M* is the total mass of the nucleus. This formula comes from substituting the reduced mass of the electron.

The Rydberg constant is one of the most precisely determined physical constants, with a relative standard uncertainty of under 2 parts in 10^{12}.^{[2]} This precision constrains the values of the other physical constants that define it.^{[7]}

Since the Bohr model is not perfectly accurate, due to fine structure, hyperfine splitting, and other such effects, the Rydberg constant cannot be *directly* measured at very high accuracy from the atomic transition frequencies of hydrogen alone. Instead, the Rydberg constant is inferred from measurements of atomic transition frequencies in three different atoms (hydrogen, deuterium, and antiprotonic helium). Detailed theoretical calculations in the framework of quantum electrodynamics are used to account for the effects of finite nuclear mass, fine structure, hyperfine splitting, and so on. Finally, the value of is determined from the best fit of the measurements to the theory.^{[8]}

The Rydberg constant can also be expressed as in the following equations.

and

where

- is the electron rest mass,
- is the electric charge of the electron,
- is the Planck constant,
- is the reduced Planck constant,
- is the speed of light in a vacuum,
- is the electrical field constant (permittivity) of free space,
- is the fine-structure constant,
- is the Compton wavelength of the electron,
- is the Compton frequency of the electron,
- is the Compton angular frequency of the electron,
- is the Bohr radius,
- is the classical electron radius.

The last expression in the first equation shows that the wavelength of light needed to ionize a hydrogen atom is 4*π*/*α* times the Bohr radius of the atom.

The second equation is relevant because its value is the coefficient for the energy of the atomic orbitals of a hydrogen atom: .

- Rydberg formula, includes a discussion of Rydberg's original discovery
- Related physical constants:

**^**Pohl, Randolf; Antognini, Aldo; Nez, François; Amaro, Fernando D.; Biraben, François; Cardoso, João M. R.; Covita, Daniel S.; Dax, Andreas; Dhawan, Satish; Fernandes, Luis M. P.; Giesen, Adolf; Graf, Thomas; Hänsch, Theodor W.; Indelicato, Paul; Julien, Lucile; Kao, Cheng-Yang; Knowles, Paul; Le Bigot, Eric-Olivier; Liu, Yi-Wei; Lopes, José A. M.; Ludhova, Livia; Monteiro, Cristina M. B.; Mulhauser, Françoise; Nebel, Tobias; Rabinowitz, Paul; Dos Santos, Joaquim M. F.; Schaller, Lukas A.; Schuhmann, Karsten; Schwob, Catherine; Taqqu, David (2010). "The size of the proton".*Nature*.**466**(7303): 213–216. Bibcode:2010Natur.466..213P. doi:10.1038/nature09250. PMID 20613837. S2CID 4424731.- ^
^{a}^{b}"2018 CODATA Value: Rydberg constant".*The NIST Reference on Constants, Units, and Uncertainty*. NIST. 20 May 2019. Retrieved 2019-05-20. **^**"2018 CODATA Value: Rydberg constant times hc in J".*NIST*. The NIST Reference on Constants, Units, and Uncertainty. Retrieved 2020-02-06.**^**"2018 CODATA Value: Rydberg constant times hc in eV".*NIST*. The NIST Reference on Constants, Units, and Uncertainty. Retrieved 2020-02-06.**^**"2018 CODATA Value: Rydberg constant times c in Hz".*NIST*. The NIST Reference on Constants, Units, and Uncertainty. Retrieved 2020-02-05.**^**Coffman, Moody L. (1965). "Correction to the Rydberg Constant for Finite Nuclear Mass".*American Journal of Physics*.**33**(10): 820–823. Bibcode:1965AmJPh..33..820C. doi:10.1119/1.1970992.**^**P.J. Mohr, B.N. Taylor, and D.B. Newell (2015), "The 2014 CODATA Recommended Values of the Fundamental Physical Constants" (Web Version 7.0). This database was developed by J. Baker, M. Douma, and S. Kotochigova. Available: http://physics.nist.gov/constants. National Institute of Standards and Technology, Gaithersburg, MD 20899. Link to R_{∞}, Link to hcR_{∞}. Published in Mohr, Peter J.; Taylor, Barry N.; Newell, David B. (2012). "CODATA recommended values of the fundamental physical constants: 2010".*Reviews of Modern Physics*.**84**(4): 1527–1605. arXiv:1203.5425. Bibcode:2012RvMP...84.1527M. doi:10.1103/RevModPhys.84.1527. S2CID 103378639""CS1 maint: postscript (link) and Mohr, Peter J.; Taylor, Barry N.; Newell, David B. (2012). "CODATA Recommended Values of the Fundamental Physical Constants: 2010".*Journal of Physical and Chemical Reference Data*.**41**(4): 043109. arXiv:1507.07956. Bibcode:2012JPCRD..41d3109M. doi:10.1063/1.4724320""CS1 maint: postscript (link).**^**Mohr, Peter J.; Taylor, Barry N.; Newell, David B. (2008). "CODATA recommended values of the fundamental physical constants: 2006".*Reviews of Modern Physics*.**80**(2): 633–730. arXiv:0801.0028. Bibcode:2008RvMP...80..633M. doi:10.1103/RevModPhys.80.633.