The quantum number which explains the line spectra observed as doublets in the case of hydrogen and alkali metals,and doublets and triplets in the case of alkaline earth metals is:

The ground state energy of a hydrogen atom is $-13.6 \ eV$. Consider an electronic state $\Psi$ of $He^+$ whose energy,azimuthal quantum number,and magnetic quantum number are $-3.4 \ eV$,$2$,and $0$ respectively. Which of the following statement$(s)$ is(are) true for the state $\Psi$?
$(1)$ It has $2$ angular nodes
$(2)$ It has $3$ radial nodes
$(3)$ It is a $4d$ state
$(4)$ The nuclear charge experienced by the electron in this state is less than $2e$,where $e$ is the magnitude of the electronic charge.

On the basis of quantum numbers,justify that the sixth period of the periodic table should have $32$ elements.

The orbital angular momentum of an electron in $2s$-orbital is

For $n = 4$,what is the total number of orbitals?

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