# NCERT Physics Class 12 Exemplar Ch 13 Nuclei Part 2

16 Why do stable nuclei never have more protons than neutrons?

Ans:

• Protons are positively charged and repel one another electrically.

• This repulsion becomes so great in nuclei with more than 10 protons or so, that an excess of neutrons which produce only attractive forces, is required for stability.

17 Consider a radioactive nucleus A which decays to a stable nucleus C through the following sequence:

Here B is an intermediate nuclei which is also radioactive. Considering that there are atoms of A initially, plot the graph showing the variation of number of atoms of A and B versus time.

Ans:

At, while. As time increases, falls off exponentially, the number of atoms of B increases becomes maximum and finally decays to zero at (following exponential decay law).

18 A piece of wood from the ruins of an ancient building was found to have a activity of 12 disintegrations per minute per gram of its carbon content. The activity of the living wood is 16 disintegrations per minute per gram. How long ago did the tree, from which the wooden sample came, die? Given half-life of is years.

Ans:

.

19 Are the nucleons fundamental particles, or do they consist of still smaller parts? One way to find out is to probe a nucleon just as Rutherford probed an atom. What should be the kinetic energy of an electron for it to be able to probe a nucleon? Assume the diameter of a nucleon to be approximately.

Ans:

• To resolve two objects separated by distance d, the wavelength λ of the proving signal must be less than d.

• Therefore, to detect separate parts inside a nucleon, the electron must have a wavelength less than m.

20 A nuclide 1 is said to be the mirror isobar of nuclide 2 if and . (a) What nuclide is a mirror isobar of Na? (b) Which nuclide out of the two mirror isobars have greater binding energy and why?

Ans:

. (a) Na :

∴ Mirror isobar of

(b) Since , Mg has greater binding energy than Na.

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