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Q.
Consider the hypothetical situation where the azimuthal quantum number, $l$, takes values $0, 1,2$,.....$ n +1$, where $n$ is the principal quantum number. Then, the element with atomic number :
Under the given situation for
$
\begin{array}{l}
n=1, l=0,1,2 \\
n=2, l=0,1,2,3 \\
n=3, l=0,1,2,3,4
\end{array}
$
According to $(n+l)$ rule of order of filling of subshells will be :
$1 s 1 p 1 d 2 s 2 p 3 s 2 d 3 f$
Atomic number $61 s^{2} 1 p^{4}$
Atomic number $91 s^{2} 1 p^{6} 1 d^{1}$
Atomic number $81 s^{2} 1 p^{6}$
Atomic number $131 s^{2} 1 p^{6} 1 d^{5}$
Therefore option (b) is correct. Atomic number of first noble gas will be $18\left(1 s^{2} 1 p^{6} 1 d^{10}\right)$.