the last shell configuration of nb atomic no.41 is 5s2,4d3......but the actual order is 5s1,4d4 ....why...what is the reason behind it

Actually, in 4d- transition series, the volm of 5s orbital is larger and so the energy difference between 4s  and 5d is much smaller so that it becomes smaller than the pairing energy of 5s orbitals. So, the 5s is not paired until the 4d orbital is full filled and its energy becomes lower than 5s orbitals. in this way the actual configuration becomes [kr]5s14d4.
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