In a Franck-Hertz experiment,an electron of energy $5.6 \ eV$ passes through mercury vapour and emerges with an energy $0.7 \ eV$. The minimum wavelength of photons emitted by mercury atoms is close to ............. $nm$.

In the absorption spectrum of $Na$,the missing wavelength$(s)$ are:

$A$ stationary hydrogen atom emits a photon corresponding to the first line of the Lyman series. The recoil velocity of the atom is nearly (in $m/s$)

The splitting of a spectral line into groups under the effect of an electric or magnetic field is called:

$A$ photon collides with a stationary hydrogen atom in the ground state inelastically. The energy of the colliding photon is $10.2 \ eV$. After a time interval of the order of a microsecond,another photon collides with the same hydrogen atom inelastically with an energy of $15 \ eV$. What will be observed by the detector?

Given below are two statements:
Statement $I$: Atoms are electrically neutral as they contain equal number of positive and negative charges.
Statement $II$: Atoms of each element are stable and emit their characteristic spectrum.
In the light of the above statements,choose the most appropriate answer from the options given below.