What is momentum of a microscopic particle having de Broglie's wavelength 6.0 A?

$$\left(\mathrm{h}=6.63 \times 10^{-34} \mathrm{Js}\right)$$

What is energy associated with fourth orbit of hydrogen atom?

$$\left(\mathrm{R}_{\mathrm{H}}=2.18 \times 10^{-18} \mathrm{~J}\right)$$

Which from following species does not have number of electrons similar to other three species?

Calculate the frequency if wavelength is $$750 \mathrm{~nm}$$.