Question

An element ‘X’ has its electronic configuration of ‘K’ shell $(n - 5)s^{2}$ and the total number of electrons in its outer-most, penultimate and antipenultimate shell are $2, 8$ and $25$ respectively, then find out the total number of unpaired electrons in element ‘X’ in its ground state

Answer 1

For $K$ shell $(n-5)s^{2}=1s^{2}$ so value of $n$ is ‘6’
Electronic configuration of ‘X ’
$1s^{2}$ ______ $4s^{2} \,\underset{2}{4p^{6}}4d^{10}\,\underset{5}{4f^{7}}5s^{2}\,\underset{8}{5p^{6}}5d^{0}\,\underset{2}{6s^{2}}$
Unpaired electron is $7$