Physics
Bohr Model
Quick fact
The Bohr model correctly predicts the wavelengths of hydrogen's spectral lines with remarkable accuracy, yet it fails for helium and other atoms with more than one electron.
Why this is interesting
If an atom were the size of a football stadium, the nucleus would be a marble at the center. So why don't the tiny electrons simply crash into it, collapsing the atom?
Read the full explanation
Understanding Bohr Model
Imagine a planet orbiting a star. In classical physics, a charged electron orbiting a nucleus should continuously radiate energy and spiral inward. Niels Bohr proposed a radical fix: electrons can only occupy certain allowed orbits with fixed energies. While in one of these orbits, the electron does not radiate and the atom is stable. An electron can jump from a higher energy orbit to a lower one by emitting a photon of exact energy, explaining the sharp lines seen in hydrogen's spectrum. Each jump produces a specific color of light, like the discrete notes on a piano.
A deeper explanation
Bohr combined Rutherford's nuclear atom with Planck's quantum idea and an ad-hoc assumption: the angular momentum of an electron is quantized in units of h/2π. This leads to orbits whose radii and energies are fixed. The model's brilliance is that it yields the Balmer formula for hydrogen's spectral lines. However, it fails for multi-electron atoms because it ignores electron-electron repulsions and the wave nature of matter. Despite its limitations, the Bohr model introduced the crucial concept of quantized energy levels, which became a cornerstone of quantum mechanics and earned Bohr the Nobel Prize in 1922.