The first quantum mechanical model of the atom was proposed by ........

Which of the following corresponds to the energy of a possible excited state of hydrogen (in $eV$)?

An excited hydrogen atom emits light in the ultraviolet region at a frequency of $2.47 \times 10^{15} \ Hz$. Calculate the energy of a single photon. (Given: $h = 6.63 \times 10^{-34} \ J \cdot s$)

The energy of an electron in the first Bohr orbit of hydrogen atom is $-2.18 \times 10^{-18} \ J$. Its energy in the third Bohr orbit is $........$.

The radius of the second orbit of a hydrogen atom is the same as that of orbit $n$ of an ion $x$. $n$ and $x$ are respectively:

The expression for Bohr's radius of an atom is