If the Aufbau principle had not been followed,$Ca$ $(Z = 20)$ would have been placed in the

Explain the energy order of orbitals in multielectron atoms.

In a multielectron atom,which of the following orbitals described by three quantum numbers will have the same energy in the absence of electric and magnetic fields?
$A: n=1, \ell=0, m_{\ell}=0$
$B: n=2, \ell=0, m_{\ell}=0$
$C: n=2, \ell=1, m_{\ell}=1$
$D: n=3, \ell=2, m_{\ell}=1$
$E: n=3, \ell=2, m_{\ell}=0$
Choose the correct answer from the options given below:

The total number of unpaired electrons in $d$-orbitals of atoms of element of atomic number $29$ is

For a single electron system,the following graph is drawn for the radial distribution function $(RDF)$ versus radial distance. The correct statement regarding the graph is:

Which among the following elements possesses one electron in the $4s$ orbital in its observed electronic configuration?