Atoms MCQs for NEET — Physics Questions with Answers

The text states: 'Classic al mechanics, based on Newton’s laws of motion, successfully describes the motion of all macroscopic objects... However it fails when applied to microscopic objects... This is mainly because of the fact that classical mechanics ignores the concept of dual behaviour of matter especially for sub-atomic particles and the uncertainty principle.' Option 3 is incorrect because classical mechanics does describe macroscopic objects successfully.

The text states: 'The reason for this was soon discovered. In Bohr model, an electron is regarded as a charged particle moving in a well defined circular orbit about the nucleus. The wave character of the electron is ignored in Bohr’s theory.'

The text says: 'The orbital picture in Bohr’s model of the hydrogen atom was inconsistent with the uncertainty principle. It was replaced by modern quantum mechanics in which Bohr’s orbits are regions where the electron may be found with large probability.'

The NCERT states, 'By performing scattering experiments in which fast electrons, instead of a-particles, are projectiles that bombard targets made up of various elements, the sizes of nuclei of various elements have been accurately measured.' This is because electrons are not subject to the strong nuclear force, allowing for more precise measurements based on electromagnetic interactions only.

The NCERT text states, 'the scattering will begin to be affected by the short range nuclear forces, and differ from Rutherford’s calculations. Rutherford’s calculations are based on pure coulomb repulsion between the positive charges of the a-particle and the gold nucleus.' This deviation from Coulomb repulsion signifies the involvement of the strong nuclear force at very close distances.

The NCERT text specifies, 'It has been found that a nucleus of mass number A has a radius $R = R_0 A^{1/3}$ where $R_0 = 1.2 \times 10^{-15} \text{ m} (=1.2 \text{ fm})$.'

The text states, 'This means the volume of the nucleus, which is proportional to $R^3$ is proportional to A.' Since $R \propto A^{1/3}$, then $R^3 \propto (A^{1/3})^3 \propto A$.

The NCERT states, 'The density of nuclear matter is approximately $2.3 \times 10^{17} \text{ kg m}^{-3}$.'

The text explains this high density by saying, 'This is understandable, as we have already seen that most of the atom is empty. Ordinary matter consisting of atoms has a large amount of empty space.' The vast emptiness of the atom means its mass is concentrated in a tiny, dense nucleus.

The NCERT states, 'As the gold foil is very thin, it can be assumed that a-particles will suffer not more than one scattering during their passage through it. Therefore, computation of the trajectory of an alpha-particle scattered by a single nucleus is enough.'