What is the number of $d$-electrons present in $Fe^{2+}$ ($Fe$ atomic number = $26$)?

An element has the electronic configuration $1s^2, 2s^2 2p^6, 3s^2 3p^2$. Its valence electrons are

The correct decreasing order of energy for the orbitals having the following set of quantum numbers:
$A$. $n=3, \ell=0, m=0$
$B$. $n=4, \ell=0, m=0$
$C$. $n=3, \ell=1, m=0$
$D$. $n=3, \ell=2, m=1$

$4s^2$ is the configuration of the outermost orbit of an element. Its atomic number would be $:-$

What is the difference between the ground state and the excited state of a hydrogen atom?

The maximum number of orbitals present in $n=4$ energy level of an atom and the maximum number of electrons with spin value $+\frac{1}{2}$ in the same orbitals are, respectively.