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The principal quantum number is the one that sets the energy level and the radial size of an orbital. In the simplest models, energy depends primarily on n (with E ~ -1/n^2 for hydrogen-like systems), and the orbital’s radial extent grows with n, so larger n means both a higher energy level and a larger orbital. The other quantum numbers describe different properties: the angular momentum quantum number (l) shapes the orbital, the magnetic quantum number (m_l) fixes its orientation, and the spin quantum number (m_s) describes the electron’s intrinsic spin. While real atoms introduce small corrections due to shielding and penetration that slightly modify energy with respect to n and l, n remains the primary descriptor of both energy level and size.