Atoms MCQs for NEET — Physics Questions with Answers

(a) Maximum energy is liberated for transition ${\mathrm{E}}_{\mathrm{n}}\to {\mathrm{E}}_1$ and minimum energy for  ${\mathrm{E}}_{\mathrm{n}}\to {\mathrm{E}}_{\mathrm{n}-1}$
Hence $\frac{{\mathrm{E}}_1}{{\mathrm{n}}^2}-{\mathrm{E}}_1=52.224 \mathrm{eV}$         ……(i)
and $\frac{{\mathrm{E}}_1}{{\mathrm{n}}^2}-\frac{{\mathrm{E}}_1}{{\left(\mathrm{n}-1\right)}^2}=1.224 \mathrm{eV}$…..(ii)
Solving equations (i) and (ii) we get
and ${\mathrm{E}}_1=-54.4 \mathrm{eV}$ and n = 5
Now ${\mathrm{E}}_1=-\frac{13.6 {\mathrm{Z}}^2}{1^2}=-54.4 \mathrm{eV}$. Hence Z = 2 

(b) As value of n increases, energy gap decreases due to increasing $Z_{eff.}$ on valence shell.

The NCERT text states: 'Bohr’s model was also unable to explain the splitting of spectral lines in the presence of magnetic field (Zeeman effect) or an electric field (Stark effect).' Options 1, 2, and 4 are phenomena that Bohr's model successfully explained for hydrogenic atoms.

The NCERT text explicitly states: 'Bohr’s model is applicable only to hydrogenic (single electron) atoms. It cannot be extended to even two electron atoms such as helium.' $H$, $He^+$, and $Li^{2+}$ are all hydrogenic species (single electron), while $He$ has two electrons.

The NCERT text mentions: 'It could not explain the ability of atoms to form molecules by chemical bonds.' This directly relates to how atoms interact and bond, which involves interatomic forces and the behavior of valence electrons.

The NCERT text states: 'An orbit is a clearly defined path and this path can completely be defined only if both the exact position and the exact velocity of the electron at the same time are known. This is not possible according to the Heisenberg uncertainty principle.' Therefore, Bohr's model, by defining fixed orbits, contradicts the uncertainty principle.

The NCERT text states: 'Bohr’s semiclassical model based on some aspects of classical physics and some aspects of modern physics also does not provide a true picture of the simplest hydrogenic atoms.'

The NCERT text mentions: 'the electron-electron electric force interaction is comparable in magnitude to the electron-nucleus electrical force, because the charges and distances are of the same order of magnitude. This is the reason why the Bohr’s model with its planet-like electron is not applicable to many electron atoms.'

The NCERT text notes: 'While the Bohr’s model correctly predicts the frequencies of the light emitted by hydrogenic atoms, the model is unable to explain the relative intensities of the frequencies in the spectrum... Experimental observations depict that some transitions are more favoured than others.' The varying intensities imply varying probabilities of transition.

The NCERT text states: 'In view of these inherent weaknesses in the Bohr model, there was no point in extending Bohr model to other atoms. In fact an insight into the structure of the atom was needed which could account for wave-particle duality of matter and be consistent with Heisenberg uncertainty principle. This came with the advent of quantum mechanics.'