U1.5 · Atomic Structure and Electron ConfigurationPractice · 原子结构与电子排布

An Fe atom is [Ar]4s²3d⁶. Cations lose electrons from the highest n first, so the 4s electrons go before any 3d: removing both 4s electrons and one 3d gives [Ar]3d⁵. Removing 3d electrons first is a common mistake.

First ionization energies (kJ/mol): O 1314, F 1681, Ne 2081, Na 496. O, F, and Ne all lose a 2p electron; Ne has the most protons with the same core shielding, so its 2p electrons are held most tightly. Na’s 3s electron is farther out and shielded by 10 core electrons, so it’s the easiest to remove.

Mn is [Ar]4s²3d⁵. The five 3d electrons spread out one per orbital (Hund’s rule), all unpaired; the 4s² pair is paired. That’s 5 unpaired electrons.

In an excited state, an electron sits above the lowest available level. 1s²2s²2p⁵3s¹ has 10 electrons; the ground state would be 1s²2s²2p⁶ (Ne), so a 2p electron has been promoted to 3s. A is ground-state Na and C is ground-state Al; D is ground-state Cr (the half-filled 4s¹3d⁵ is a stable exception to the aufbau order, not an excited state).

By Coulomb’s law, the attraction depends on nuclear charge and distance. A 1s electron is in the innermost shell and barely shielded: Mg has 12 protons to Na’s 11, so its 1s electrons are attracted more strongly and sit a little closer, needing more energy to remove. More repulsion would make removal easier, the opposite of what’s observed, and valence electrons don’t affect the 1s electrons.