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2.85 In the heavier transition-metal elements, especially the lanthanoids and actinoids, there are numerous exceptions to the regular order of orbital occupation predicted by the building- up principle. Suggest why more exceptions would be noted for there elements.
In the heavier transition-metal elements, the energy levels of the subshells get so close that their energy levels actually depend on how many electrons are occupying them. For example, 4s is filled before 3d because 4s has a lower energy level (rather than 3d filling before 4s like we might normally expect from the building-up principle)
In general, we don't really need to worry about heavier transition-metal elements because they don't usually follow normal patterns. But in general, the prime reason for their behavior will be their abnormally large size because they have so many more protons and electrons than elements in the first few rows.
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