spectra

Meaning: An atom's electrons occupy quantized energy levels. When an electron drops to a lower level, the atom emits a photon whose energy equals the gap. When it absorbs a photon of exactly that energy, the electron moves up. Gaps are fixed for each element, so the result is discrete lines, not a continuous band. A hot, dense solid gives a continuous spectrum instead. (In organic chemistry, IR, UV-vis and NMR spectra are used to deduce structure.)

Equation: ΔE = Ephoton = hf = hc/λ, where ΔE is the level gap in J, h is Planck's constant (6.63 × 10⁻³⁴ J·s), f is frequency in Hz, c is 3.00 × 10⁸ m/s, and λ is wavelength in m. For hydrogen, En = −13.6 eV/n², where n is the principal quantum number.

On the MCAT: A passage gives an energy level diagram or a set of spectral lines. You rank transitions by photon energy: a larger gap gives a higher frequency and a shorter wavelength. You may also match lines to transitions, or compare emission and absorption spectra.

Common confusion: Emission lines (bright lines on a dark background) and absorption lines (dark lines on a bright background) for the same element fall at the same wavelengths. Also, a higher n does not mean a larger gap. Levels crowd together as n increases, so n = 2 → 1 releases more energy than 3 → 2 or 4 → 3.