Atoms MCQs for NEET — Physics Questions with Answers

The text mentions, 'From kinetic theory, the size of an atom was known to be $10^{-10} \text{ m}$'.

The NCERT states, 'Alpha-particles are nuclei of helium atoms and, therefore, carry two units, 2e, of positive charge and have the mass of the helium atom.'

The NCERT explains, 'It is seen that an a-particle close to the nucleus (small impact parameter) suffers large scattering. In case of head-on collision, the impact parameter is minimum and the a-particle rebounds back (q @ p).' A small impact parameter means the alpha-particle passes very close to the nucleus, experiencing a strong repulsive force and thus a large deflection.

The NCERT states, 'The fact that only a small fraction of the number of incident particles rebound back indicates that the number of a-particles undergoing head on collision is small. This, in turn, implies that the mass and positive charge of the atom is concentrated in a small volume.'

The text states, 'The radius of the atom is about $10^{-10} \text{ m}$, while that of nucleus is $10^{-15} \text{ m}$. One can appreciate this difference in size by realising that if Fig. 2.5 Schematic view of Rutherford’ s scattering experiment.' And also, 'Rutherford’s experiments suggested the size of the nucleus to be about $10^{-15} \text{ m}$ to $10^{-14} \text{ m}$. From kinetic theory, the size of an atom was known to be $10^{-10} \text{ m}$, about 10,000 to 100,000 times larger than the size of the nucleus.'

The NCERT text explains, 'The atomic electrons, being so light, do not appreciably affect the a-particles.'

The text states, 'This agreement supported the hypothesis of the nuclear atom. This is why Rutherford is credited with the discovery of the nucleus.' This hypothesis was that 'the greater part of the mass of the atom and its positive charge were concentrated tightly at its centre.'

This is taken from 'EXERCISES 12.2 Suppose you are given a chance to repeat the alpha-particle scattering experiment using a thin sheet of solid hydrogen in place of the gold foil. (Hydrogen is a solid at temperatures below 14 K.) What results do you expect?' The key insight is that hydrogen has only one proton (Z=1), meaning its nucleus is much smaller and has a much weaker electric field compared to gold (Z=79). Therefore, the repulsive force on the alpha particles ($2e$ charge) would be significantly less, and most alpha particles would pass through with minimal or no deflection.

$\lambda_{\min} = \dfrac{hc}{eV}\Rightarrow \lambda_{\min}\propto \dfrac{1}{V}$.

Series limit: $1/\lambda_{\min} = R/n^2$. Balmer ($n=2$): $1/\lambda = R/4$. Bracket ($n=4$): $1/\lambda' = R/16$. So $\lambda' = 4\lambda$.