Which physical constant named for Bohr measures an atomic length scale?

The physical constant named for Bohr that measures an atomic length scale is the Bohr radius.

Usually written as a₀, the Bohr radius is approximately 5.29 × 10⁻¹¹ metres, or 52.9 picometres. In the simplest model of hydrogen, it is the radius of the lowest-energy electron orbit. In modern quantum mechanics, it corresponds closely to the most probable radial distance of the electron from the proton in hydrogen’s ground state.

The constant comes from Niels Bohr’s 1913 model of the atom, which introduced quantized electron energy levels. Although that orbit picture was later replaced by wave mechanics, the Bohr radius remains fundamental in atomic physics and is the atomic unit of length.

The alternatives describe different concepts. The Rydberg constant concerns spectral wavenumbers, the Compton wavelength is associated with quantum wavelength effects, and the Planck length is a vastly smaller scale linked to quantum gravity.