Number of unpaired electrons in $Fe^{+3}$ $(Z = 26)$ is

Compare the energies of the following sets of quantum numbers for a multielectron system:
$A. n=4, \ell=1$
$B. n=4, \ell=2$
$C. n=3, \ell=1$
$D. n=3, \ell=2$
$E. n=4, \ell=0$
Choose the correct answer from the options given below:

The electronic configuration of an element is $1s^2 2s^2 2p^6 3s^2 3p^6 4s^1$. This represents:

$4d, 5p, 5f$ and $6p$ orbitals are arranged in the order of decreasing energy. The correct option is

Explain the energy order of orbitals in multielectron atoms.

The four quantum numbers for the valence shell electron or last electron of sodium $(Z = 11)$ are: