orbits

Meaning: In the Bohr model, an electron moves in a circular orbit around the nucleus, and only certain orbits are allowed. Each is labeled n = 1, 2, 3, … and has a fixed energy, so energy is quantized. Energy is negative relative to a free electron (E = 0 at n = ∞), so larger n means a larger radius and higher energy. The model works well for hydrogen and other one-electron species. (In physics, orbits also describes satellites and planets under gravity.)

Equation: En = −(2.18 × 10⁻¹⁸ J)/n², where En is the electron's energy in joules and n is the principal quantum number (a positive integer, unitless), for hydrogen.

On the MCAT: Passages show hydrogen emission or absorption lines and ask which transition gives the highest-energy photon or longest wavelength. Use ΔE = hf = hc/λ. A larger gap between orbits means higher frequency and shorter wavelength. Gaps shrink as n rises, so 2 → 1 releases more energy than 3 → 2.

Common confusion: Orbits are not orbitals. An orbit is a fixed path; an orbital is a region of probability described by quantum numbers, because the uncertainty principle rules out a definite path. Also, a photon is absorbed only if its energy matches an orbit gap exactly.