4. When iron is oxidized, loses electrons, why does the electron come from the valence shell (45) instead of the highest energy shell (3d)?

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When electrons are removed, they are always removed from the outermost or valence shell. Because the outer shell is maximum shielded from the influence of the nucleus, and hence lower energy is required to extract electrons from the outermost shell than from an inner shell. Since 4s orbital lies in the fourth shell, and 3d in the third shell, So, 4s electrons being shielded from the nucleus by the inner 3d orbitals. Hence electrons are removed from 4s orbitals insted of 3d orbitals. So when iron oxidizes electrons are removed from 4s orbitals first.