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

‘l’ can have values (n-1) to (n+1) total n+2 energy levels$\begin{array}{llll}1 \mathrm{s}& 1\mathrm{p}& 1 \mathrm{d}& \\ 2 \mathrm{s}& 2\mathrm{p}& 2 \mathrm{d}& 2\mathrm{f}\end{array}$

         1) Atomic number 18 is having electronic configuration  $1{\mathrm{s}}^{2}1{\mathrm{p}}^{6}1{\mathrm{d}}^{10}$ and is noble gas.

          2) Atomic number ‘19’ is first alkali metal

          3) Atomic number ‘6’ has  1s, 1p valence subshell.

          4) atomic number 16 has $1{\mathrm{s}}^{2}1{\mathrm{p}}^{6}1{\mathrm{d}}^5$  halffilled d-configuration