What Is The Rydberg Formula?

Understand the Rydberg Equation

The Rydberg formula can be used to predict the spectral line wavelengths of the elements.
The Rydberg formula can be used to predict the spectral line wavelengths of the elements. Roman Okopny / Getty Images

Rydberg Formula Definition

The Rydberg formula is a mathematical formula used to predict the wavelength of light resulting from an electron moving between energy levels of an atom.

When an electron changes from one atomic orbital to another, the electron's energy changes. When the electron changes from an orbital with high energy to a lower energy state, a photon of light is created. When the electron moves from low energy to a higher energy state, a photon of light is absorbed by the atom.

Each element has a distinct spectral fingerprint. When an element's gaseous state is heated, it will give off light. When this light is passed through a prism or diffraction grating, bright lines of different colors can be distinguished. Each element is slightly different from other elements. This discovery was the beginning of the study of spectroscopy.

Rydberg Formula Equation

Johannes Rydberg was a Swedish physicist who attempted to find a mathematical relationship between one spectral line and the next of certain elements. He eventually discovered there was an integer relationship between the wavenumbers of successive lines.

His findings were combined with Bohr's model of the atom to give the formula:

1/λ = RZ2(1/n12 - 1/n22)

λ is the wavelength of the photon (wavenumber = 1/wavelength)
R = Rydberg's constant (1.0973731568539(55) x 107 m-1)
Z = atomic number of the atom
n1 and n2 are integers where n2 > n1.

It was later found n2 and n1 were related to the principle quantum number or energy quantum number. This formula works very well for transitions between energy levels of a hydrogen atom with only one electron. For atoms with multiple electrons, this formula begins to break down and give results that are incorrect.

The reason for the inaccuracy is that the amount of screening for inner electrons for outer electron transitions varies. The equation is too simplistic to compensate for the differences.

The Rydberg formula may be applied to hydrogen to obtain its spectral lines. Setting n1 to 1 and running n2 from 2 to infinity yields the Lyman series. Other spectral series may also be determined:

n1n2Converges TowardName
12 → ∞91.13 nm (ultraviolet)Lyman series
23 → ∞364.51 nm (visible light)Balmer series
34 → ∞820.14 nm (infrared)Paschen series
45 → ∞1458.03 nm (far infrared)Brackett series
56 → ∞2278.17 nm (far infrared)Pfund series
67 → ∞3280.56 nm (far infraredHumphreys series

For most problems, you'll deal with hydrogen so you can use the formula:

1/λ = RH(1/n12 - 1/n22)

where RH is Rydberg's constant, since the Z of hydrogen is 1.

Rydberg Formula Worked Example Problem

Find the wavelength of the electromagnetic radiation that is emitted from an electron relaxes from n = 3 to n = 1.

To solve the problem, start with the Rydberg equation:

1/λ = R(1/n12 - 1/n22)

Now plug in the values, where n1 is 1 and n2 is 3. Use 1.9074 x 107 m-1 for Rydberg's constant:

1/λ = (1.0974 x 107)(1/12 - 1/32)
1/λ = (1.0974 x 107)(1 - 1/9)
1/λ = 9754666.67 m-1
1 = (9754666.67 m-1
1 / 9754666.67 m-1 = λ
λ = 1.025 x 10-7 m

Note the formula gives a wavelength in meters using this value for Rydberg's constant. You'll often be asked to provide an answer in nanometers or Angstroms.

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Helmenstine, Todd. "What Is The Rydberg Formula?" ThoughtCo, Jun. 6, 2017, thoughtco.com/what-is-the-rydberg-formula-604285. Helmenstine, Todd. (2017, June 6). What Is The Rydberg Formula? Retrieved from https://www.thoughtco.com/what-is-the-rydberg-formula-604285 Helmenstine, Todd. "What Is The Rydberg Formula?" ThoughtCo. https://www.thoughtco.com/what-is-the-rydberg-formula-604285 (accessed November 22, 2017).