An atom has $2$ electrons in $K$ shell,$8$ electrons in $L$ shell and $6$ electrons in $M$ shell. The number of $s$-electrons present in that element is

The electronic configuration of cobalt ($Co$,atomic number $27$) is:

Which of the following statement$(s)$ is/are true?
$A$. If two orbitals have the same value of $(n+l)$,the orbital with lower value of $n$ will have lower energy.
$B$. Energies of the orbitals in the same subshell increase with increase in atomic number.
$C$. The size of $2p_x$ orbital is less than the size of $3p_x$ orbital.
$D$. Among $5f, 6s, 4d, 5p$ and $5d$ orbitals,none of the orbitals have $2$ radial nodes.

Which one of the following sets of quantum numbers represents an impossible arrangement?

For a one-electron atom,the set of allowed quantum numbers is

Identify an orbital with the quantum numbers $n=4, \ell=3, m=0$.