Paramagnetic substances must contain .....

The rule which states that an electron enters the sub-shell for which the $(n + l)$ value is minimum is known as:

In the case of a hydrogen atom,which of the following transitions does not involve the absorption or emission of energy?

Which of the following has the maximum number of unpaired electrons? (Atomic number of $Fe = 26$)

$A$: No two electrons in an atom can have the same set of all four quantum numbers.
$R$: Two electrons in an atom can exist in the same shell,subshell,or orbital only if they have opposite spins.

The configuration $1s^2 2s^2 2p^5 3s^1$ shows: