Detailed Breakdown: Electron Configuration
The Quantum Mechanical Model of the Atom: Electrons do not move in simple orbits; they occupy orbitals—regions of space where there is a high probability of finding an electron. These orbitals are organized into subshells ($s, p, d, f$) and principal energy levels ($n=1, 2, 3\dots$).
Governing Principles of Electron Arrangement: The Aufbau Principle (fill lowest energy first), the Pauli Exclusion Principle (max 2 electrons per orbital with opposite spins), and Hund's Rule (fill degenerate orbitals singly first) together predict the chemical reactivity and magnetic properties of an element.
Atomic Spectra and Quantification: When electrons transition between energy levels, they absorb or emit light of specific frequencies. This produces line spectra (not continuous), proving that energy levels in atoms are quantized. For example, the Lyman series (UV) represents transitions to the $n=1$ level.