Question

If 20 electrons are present in the 4th orbit then the atomic number of that element is

Answer 1

If there are 20 electrons in the 4th orbit then it suggests that the 4th orbit has electrons filled as 4s2 4p6 4d10 4f2 , and as per aufbau principle and hunds rule the element would belong to the inner transition series and would have electronic configufation as 1s2.2s2.2p6.3s2.3p6.4s2.3d10.4p6.5s2.4d10.5p6.6s2.4f2 or else simply [Xe] 4f2 6s2 .

But in the Lanthanum series we don’t have any element that has this configuration. Because the electronic configuration of cerium is [Xe] 6s2 4f1 5d1 hence has only 19 electrons in its 4th orbit while the next element Praseodymium has electronic configuration as [Xe]6s2 4f3 5d1 hence it has 21 electron in its 4th orbit.

But still if you only need the atomic number of the element it would be 58 i.e Cerium but as energy difference between 4f and 5d is very small the electron from 4f enters 5d orbital and hence we don’t have any element which has electronic configuration of 4f2 hence under normal conditions and in ground state we have no element which has 20 electrons in its 4th orbit but in excited state of +2, cerium has this electronic configuration hence in excited state the element would be cerium with atomic number 58.

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Answer 2

Answer:

At no. is 59

Explanation:

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