According to Bohr's theory, the moment of momentum of an electron revolving in $4^{\text {th }}$ orbit of hydrogen atom is:

<p>According to Bohr's theory, one of the postulates specifies that the angular momentum of an electron in orbit around a nucleus is quantized. This quantization can be expressed by the formula:</p> <p>$L = n\frac{h}{2\pi}$</p> <p>Where:</p> <ul> <li>$L$ is the angular momentum of the electron,</li> <li>$n$ is the principal quantum number (or the orbit number in simpler terms), which can take positive integer values (1, 2, 3, ...),</li> <li>$h$ is Planck's constant ($6.62607015 \times 10^{-34} m^2 kg / s$), and</li> <li>$\frac{h}{2\pi}$ is often denoted as $\hbar$ (h-bar), known as the reduced Planck's constant.</li> </ul> <p>For an electron in the 4th orbit ($n = 4$) of a hydrogen atom, we substitute $n = 4$ into the equation:</p> <p>$L = 4\frac{h}{2\pi}$</p> <p>Therefore, the moment of momentum (or angular momentum) of an electron in the $4^{\text{th}}$ orbit of a hydrogen atom is:</p> <p>$L = 4\frac{h}{2\pi} = 2\frac{2h}{2\pi} = 2\frac{h}{\pi}$</p> <p>Hence, the correct option is:</p> <p>Option A: $2 \frac{h}{\pi}$</p>