Which of the following transitions are allowed in the normal electronic emission spectrum of an atom?

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

Quantum numbers $l = 2$ and $m = 0$ represent which orbital?

Consider the following pairs of electrons:
$(A)$ $(a)$ $n=3, l=1, m_{l}=1, m_{s}=+\frac{1}{2}$
$(b)$ $n=3, l=2, m_{l}=1, m_{s}=+\frac{1}{2}$
$(B)$ $(a)$ $n=3, l=2, m_{l}=-2, m_{s}=-\frac{1}{2}$
$(b)$ $n=3, l=2, m_{l}=-1, m_{s}=-\frac{1}{2}$
$(C)$ $(a)$ $n=4, l=2, m_{l}=2, m_{s}=+\frac{1}{2}$
$(b)$ $n=3, l=2, m_{l}=2, m_{s}=+\frac{1}{2}$
The pair$(s)$ of electrons present in degenerate orbitals is/are...... .

An element has the electronic configuration $1s^2, 2s^2 2p^6, 3s^2 3p^6 3d^5, 4s^1$. It is a

If $n$ and $l$ are principal and azimuthal quantum numbers respectively,then the expression for calculating the total number of electrons in any energy level is: