What is de Broglie's wavelength for a particle having mass $6.64 \times 10^{-27} \mathrm{~kg}$ moving with velocity of $3 \times 10^3 \mathrm{~ms}^{-1} ?\left[\mathrm{~h}=6.63 \times 10^{-34} \mathrm{Js}\right]$

Which of the following series of emission spectral lines for hydrogen observed in visible region?

What is wavenumber of a radiation having wavelength $0.25 \mu \mathrm{~m}$ ?

Which of the following species contain 20 electrons?