NEET Practice Questions (MCQs) with Answers & Solutions

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.'

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.