Which of the following represents the velocity of an electron in a hydrogen-like atom?

Observe the following statements:
Statement-$I$: Rutherford model of an atom cannot explain the stability of an atom.
Statement-$II$: The wavelength of $X$-rays is higher than the wavelength of microwaves.
The correct answer is:

The quantum theory was first used to explain the structure of the atom,which was proposed by .......

The frequency of radiation emitted when the electron falls from $n = 4$ to $n = 1$ in a hydrogen atom will be (Given $h = 6.625 \times 10^{-34} \, J s$):-

Positronium consists of an electron and a positron (a particle which has the same mass as an electron,but opposite charge) orbiting round their common centre of mass. Calculate the value of the Rydberg constant for this system.

The magnitude of kinetic energy $(K.E.)$ of an electron in a Bohr orbit is equal to which of the following?