Periodic Table 2.85

[Semi Yoon]

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. Why would more exceptions would be noted for these elements?

[Anushi Patel 1J]

Because as energy levels increase, the energy difference between one and the next decreases. This means there is less of a clear distinction between which subshell has a lower energy and therefore should be filled first.