When $\alpha$-particles are sent through a thin metal foil,most of them go straight through the foil because

The wavelength of the first line of the Balmer series in the hydrogen spectrum is ................ $\mathring{A}$.

Ionisation energy of $He^{+}$ is $19.6 \times 10^{-18} \, J \, atom^{-1}$. The energy of the first stationary state $(n = 1)$ of $Li^{2+}$ is

The Balmer series in the hydrogen spectrum corresponds to the transition from $n_1 = 2$ to $n_2 = 3, 4, ...$ This series lies in the visible region. Calculate the wave number of the line associated with the transition in the Balmer series when the electron moves to the $n = 4$ orbit. $(R_H = 109677 \ cm^{-1})$

The electron in the $n^{th}$ orbit of $Li^{2+}$ is excited to $(n+1)$ orbit using the radiation of energy $1.47 \times 10^{-17} \ J$. The value of $n$ is $....$. Given $R_H = 2.18 \times 10^{-18} \ J$.

For which of the following species,Bohr's theory is not applicable?