The number of unpaired electrons in a ferrous ion $(Fe^{2+})$ is:

$A$ completely filled $d$ orbital $(d^{10})$ configuration 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 of the following statements are true?
$(A)$ The subsidiary quantum number $\ell$ describes the shape of the orbital occupied by the electron.
$(B)$ The image shows the boundary surface diagram of the $2p_x$ orbital.
$(C)$ The $+$ and $-$ signs in the wave function of the $2p_x$ orbital refer to charge.
$(D)$ The wave function of the $2p_x$ orbital is zero everywhere in the $yz$ plane.

According to the Aufbau principle,what is the correct order of energy for $3d$,$4s$,and $4p$ orbitals?

Krypton $(_{36}Kr)$ has the electronic configuration $[Ar] 4s^2 3d^{10} 4p^6$. The $37^{th}$ electron will go into which of the following subshells?