Q.27) De-Broglie wavelength of an electron orbiting in the $n=2$ state of hydrogen atom is close to $($ Given Bohr radius $=0.052 \mathrm{~nm})$
A) 2.67 nm
B) 0.167 nm
C) 0.67 nm
D) 1.67 nm
Solution:
Correct Option: (3) 0.67 nm
In the Bohr model, the radius of the $n$-th orbit is given by $r_n=n^2 a_0$. The de-Broglie wavelength is then:
$$
\lambda=\frac{2 \pi r_n}{n}=\frac{2 \pi n^2 a_0}{n}=2 \pi a_0 n
$$
Substituting $a_0=0.052 \mathrm{~nm}$ and $n=2$ :
$$
\lambda=2 \cdot \pi \cdot 0.052 \cdot 2=0.104 \cdot \pi \approx 0.104 \cdot 3.14 \approx 0.65 \mathrm{~nm}
$$
Which is approximately 0.67 nm .
Hence, the answer is option (3).