For principal quantum number $n = 4$,the total number of orbitals having $l = 3$ is:

While filling up of electrons in the atomic orbitals,the $4s$ orbital is filled before the $3d$ orbital but reverse happens during the ionisation of the atom. Explain why?

The set of quantum numbers not applicable for an electron in an atom is

Which of the following orbitals are possible: $1p, 2s, 2p, 3f$?

$A$ hydrogen atom has only one electron,so mutual repulsion between electrons is absent. However,in multielectron atoms,mutual repulsion between the electrons is significant. How does this affect the energy of an electron in the orbitals of the same principal quantum number in multielectron atoms?

Which electron is associated with the least energy?