The ratio between kinetic energy and the total energy of the electrons of a hydrogen atom according to Bohr's model is

The element used by Rutherford in his famous scattering experiment was

The work functions of two metals ($M_A$ and $M_B$) are in the $1:2$ ratio. When these metals are exposed to photons of energy $6 \ eV$,the kinetic energy of liberated electrons of $M_A:M_B$ is in the ratio of $2.642:1$. The work functions (in $eV$) of $M_A$ and $M_B$ are respectively.

One energy difference between the states $n = 2$ and $n = 3$ is $E \ eV$,in a hydrogen atom. The ionization potential of the $H$ atom is ............. $E$.

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$:
Assertion $A$: In the photoelectric effect,the electrons are ejected from the metal surface as soon as the beam of light of frequency greater than threshold frequency strikes the surface.
Reason $R$: When the photon of any energy strikes an electron in the atom,transfer of energy from the photon to the electron takes place.
In the light of the above statements,choose the most appropriate answer from the options given below:

The radius of the first Bohr orbit of the hydrogen atom is the same as that of the orbit $(n)$ of a hydrogen-like species $X$. The values of $(n)$ and $X$ respectively are: