ground state

Meaning: The ground state is the lowest-energy arrangement of an atom's electrons. Electrons fill from the lowest orbital upward (Aufbau), hold at most two per orbital with opposite spins (Pauli), and fill degenerate orbitals singly first (Hund). Absorbing a photon matching an energy gap gives an excited state; falling back emits a photon. For hydrogen, the ground state is n = 1.

Equation: En = −2.18 × 10⁻¹⁸ J / n² (hydrogen), where En is the electron's energy (J) and n is the principal quantum number (unitless). Photon: ΔE = hf = hc/λ, where h = 6.63 × 10⁻³⁴ J·s, f is frequency (Hz), c = 3.00 × 10⁸ m/s, and λ is wavelength (m).

On the MCAT: Expect a configuration to be labeled ground or excited, a comparison of photon energies for different transitions, or the energy needed to ionize hydrogen from n = 1 (2.18 × 10⁻¹⁸ J). Larger gaps mean higher-frequency, shorter-wavelength light.

Common confusion: Ground-state energy is the most negative value, not zero; zero is the free electron at n = ∞. Fluorine's 1s²2s²2p⁴3s¹ is excited because a lower orbital is not full. A few elements, such as Cr and Cu, have ground states that depart from the simple filling order.