JEE Main 2020 — Structure of Atom Question with Solution

For $\mathrm{n}=1$   value of $\mathcal{l}=0, 1, 2$

For $\mathrm{n}=2$   value of $\mathcal{l}=0, 1, 2, 3$

So, according to $\mathrm{n}+\mathrm{l}$ rule the filling order of subshells will be :

$1\mathrm{s} 1\mathrm{p} 2\mathrm{s} 1\mathrm{d} 2\mathrm{p} 3\mathrm{s} 2\mathrm{d} 3\mathrm{p} 4\mathrm{s}.....$

(1) $1^{\mathrm{st}}$ alkali metal will have configuration $1{\mathrm{s}}^2 1{\mathrm{p}}^6$ so atomic number will be $8$.

(2) $1^{\mathrm{st}}$ alkali metal will have electronic configuration $\Rightarrow 1{\mathrm{s}}^1 \Rightarrow \left(\mathrm{Z}=1\right)$

(3) Electronic configuration of $\mathrm{C}\left(\mathrm{Z}=6\right) \Rightarrow 1{\mathrm{s}}^2 1{\mathrm{p}}^4$

(4) $\mathrm{Z}=13$, Electronic configuration $=1{\mathrm{s}}^2 1{\mathrm{p}}^6 2{\mathrm{s}}^2 1{\mathrm{d}}^3$

So it will not have half-filled electronic configuration.