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Whenever there are 4 unpaired electrons in d-orbital, elements try to attain stable electronic configuration. For example copper.
Electronic configuration of copper should be [Ar]3d4 4s2, but it tries to attain stable state and so it changes to [Ar]3d5 4s1.
I have seen it happens with most of the elements but not with tungsten. Electronic configuration of tungsten is [Xe]4f14 5d4 6s2 and not [Xe]4f14 5d5 6s1(more stable).
What is the reason behind that?