Bohr Radius

SYMBOL: a₀
STATUS: Measured (CODATA 2022)
VALUE: 5.29177210544 × 10⁻¹¹ ± 8.2 × 10⁻²¹ m

The most probable distance between proton and electron in ground-state hydrogen, about half an ångström.

Equals 5.29 × 10⁻¹¹ m — the radius scale of every atom's electron cloud.
In Bohr's 1913 model it was the ground-state orbit radius; in quantum mechanics, the most probable electron-proton distance in hydrogen.
Equals ħ divided by mₑ·c·α, so it would shrink if the electron were heavier or electromagnetism stronger.