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Get 100 percent accurate NCERT Solutions for Class 12 Physics Chapter Atoms and Nuclei solved by expert Physics teachers. We provide solutions for questions as per CBSE Board guidelines from the latest NCERT book for Class 12 Physics.

NCERT Grade 12 Physics from Unit 8, Atoms and Nuclei. In this chapter, subsequent studies on atoms that showed the distribution of the electrons and positive charges are very different from that proposed in J.J. Thomson model of 1898 are described. Also, since hydrogen is the simplest of the elements known, its spectrum in detail is considered in this chapter.

NCERT Grade 12 Physics Chapter 12, Atoms is from Unit 8, Atoms and Nuclei. Unit 8 holds a total weightage of 6 marks in the final examination.

Download CBSE Class 12th Standard CBSE Physics question papers, sample papers, important questions, and previous year solved papers in PDF format. Get free study materials, NCERT solutions, and exam preparation resources for Class 12th Standard CBSE Physics

Questions:

Generic 3

3 x 3 = 9

Calculate the wavelength of photon emitted when an electron jumps from 3^rd orbit to 2nd orbit. Take \(R=1.097\times { 10 }^{ 7 }{ m }^{ -1 }\). Is this photon visible?

An alpha particle of energy 4MeV is scattered through \(180°\) by a gold foil (Z = 79). Calculate the maximum volume in which positive charge of the atom is likely to be concentrated?

'The half life \(_{ 6 }^{ 14 }{ C }\) is 5700 years.' What does it mean ?
Two radioactive nuclei X and Y initially contain an equal number of atoms. Their half life is 1 hour and 2 hours respectively. Calculate the ratio of their rates of disintegration after two hours.

Multiple Choice Question

7 x 1 = 7

A radioisotope X with a half life \(1.4\times { 10 }^{ 9 }\) years decays to Y, which is stable. A sample of the rock from a cave was found to contain X and Y in the ratio 1 : 7. The age of the rock is

(a)

\(1.96\times { 10 }^{ 9 }\ years\)

(b)

\(3.92\times { 10 }^{ 9 }\ years\)

(c)

\(4.20\times { 10 }^{ 9 }\ years\)

(d)

\(8.40\times { 10 }^{ 9 }\ years\)

If a star can convert all the He nuclei completely into oxygen nuclei, the energy released per oxygen nucleus is [Mass of He nucleus is 4.0026 amu and mass of Oxygen nucleus is 15.9994 amu]

(a)

7.6 MeV

(b)

56.12 MeV

(c)

10.24 MeV

(d)

23.9 MeV

Consider a hydrogen atom with its electron in the nth orbit. An electromagnetic radiation of wavelength 90 nm is used ionize the atom. If the kinetic energy of the ejected electron is 10.4 eV, then the value of n is (h_c = 1242 eV nm)

(a)

(b)

(c)

(d)

The series of hydrogen spectrum which lies in visible region is

(a)

Lyman series

(b)

Balmer series

(c)

Paschen series

(d)

none of the above

Consider aiming a beam of free electrons towards free protons. When they scatter, an electron and a proton cannot combine to produce a H-atom,

(a)

because of energy conservation

(b)

without simultaneously releasing energy in the form of radiation

(c)

because of momentum conservation

(d)

because of angular momentum conservation.

For the ground state, the electron in the H-atom has an angular momentum =h, according to the simple Bohr model. Angular momentum is a vector and hence there will be infinitely many orbits with the vector pointing in all possible directions. In actuality, this is not true,

(a)

because only one of these would have a minimum energy

(b)

because only one of these would have a minimum energy

(c)

angular momentum must be in the direction of spin of electron

(d)

because electrons go arround only in horizontal obits.

10.

Taking the Bohr radius as a₀ = 53pm, the radius of Li⁺⁺ ion its ground state, on the basis of Bohr's model, will be about

(a)

53pm

(b)

27pm

(c)

18pm

(d)

13pm

Generic 2

10 x 2 = 20

11.

Nuclear fusion is not possible in laboratory.Why?

12.

Why is \(_{ 92 }{ { U }^{ 238 } }\) not suitable for chain reaction?

13.

Give the order of magnitude of nuclear mass density and average atomic mass density. Compare these densities with the typical mass density of solids, liquids, and gases (at ordinary temperature and pressure).

14.

Calculate the energy equivalent of 1 a.m.u. in MeV.

15.

The mean life of a radioactive sample is \({ T }_{ m }\). What is the time in which 50% of the sample would get decayed?

16.

How many coulombs of charge is carried by 1kg of electrons?

17.

What is the impact parameter for scattering of \(\alpha \)- particle by \(180°\)?

18.

Name the series of hydrogen spectrum which lies in the visible region of electromagnetic spectrum?

19.

Name the series of hydrogen spectrum, which has least wavelength.

20.

How is impact parameter related to the scattering angle?

Generic 5

2 x 5 = 10

21.

On the basis of Bohr's theory of H-atom, total energy of electron in a stationary orbit is given by the relation
Where, n is the number of orbits. It is clear that total energy of electron in a stationary orbit is negative which implies that electron is bound to the nucleus and is not free to leave it. The value of negative energy decreases as n increases.
Read the above p[assagr and answer the following questions:
(i) What is the total energy of electron in ground state of hydrogen atom? What does it indicate?
(ii) Why does energy required to remove an electron is smaller when atom is in any one excited state?

22.

A nuclear reactor is a powerful device, wherein nuclear energy is utilised for peaceful purposes. It is based upon controlled nuclear chain reaction. The nuclear chain reaction is controlled by the use of control rods (of boron or cadmium) and moderators like heavy water, graphite, etc. The whole reactor is protected with concrete walls 2 to 2.5 metre thick, so that radiations emitted during nuclear reactions may not produce harmful effects.
Read the above passage and answer the following questions:
(i) Give any two merits of nuclear reactors.
(ii) What is radiactive waste?
(iii) Why do people often oppose the location site of a nuclear reactor? What do you suggest?

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Answers Key:

Generic 3

3 x 3 = 9

\(6563\mathring { A } ,Yes\)

\(7.7\times { 10 }^{ -40 }{ m }^{ 3 }\)
Maximum volume in which positive charge of atom is likely to be concentrated is
\({ V }_{ 0 }=\frac { 4 }{ 3 } \pi { r }_{ 0 }^{ 3 }\)
Calculate \({ r }_{ 0 }\) from the usual formula.

1 : 1

Multiple Choice Question

7 x 1 = 7

(b)
2

(b)
Balmer series

(b)
without simultaneously releasing energy in the form of radiation

(a)
because only one of these would have a minimum energy

10.

Generic 2

10 x 2 = 20

11.

This is because nuclear fusion requires temperatures as high as \({ 10 }^{ 6 }-{ 10 }^{ 7 }\ K\). Such high temperature is often generated in nuclear fission. That is why fission proceeds fusion. These processes cannot be carried out in laboratory.

12.

This is because only fast moving neutrons of 12 MeV energy can cause fission of \(_{ 92 }{ { U }^{ 238 } }\) nuclei. Such neutrons have less chance of interaction. They escape the fissionable material without causing fission.

13.

Nuclear mass density is of the order of \(PFP-V-II-91/11\). This is \({ 10 }^{ 13 }\) to \({ 10 }^{ 14 }\) times the average atomic mass density, which is of the order of \({ 10 }^{ 3 }\) to \({ 10 }^{ 4\ }kg{ m }^{ -3 }\). Typical mass densities of solids and liquids are of the same order as the atomic mass densities. This is because the atoms are tightly packed in these phases. The typical densities of gases at S.T.P are of the order of \({ 10 }^{ -1 }\ kg{ m }^{ -3 }\) to \(1\ kg{ m }^{ -3 }\).

14.

From \(E=m{ c }^{ 2 }\)
when m=1 a.m.u = \(1.66\times { 10 }^{ -27 }kg\)
\(E=(1.66\times { 10 }^{ -27 }){ \left( 3\times { 10 }^{ 8 } \right) }^{ 2 }joule\)
\(=\frac { 1.66\times { 9\times 10 }^{ -11 } }{ 1.6\times { 10 }^{ -13 } } MeV=933.75MeV\)

15.

Time in which 50% of the sample will get decayed is half life
\(T=\frac{0.693}{\lambda}=0.693 \tau=0.693 T_m\)

16.

\(1.77 \times 10^{11} \mathrm{C}.\) It is the value of e/m of electrons.

17.

Zero. This follows from the relation \(b=\frac { Z{ e }^{ 2 }cot\quad { \theta }/{ 2 } }{ 4\pi { \epsilon }_{ 0 }\left( \frac { 1 }{ 2 } m{ v }^{ 2 } \right) } \)

18.

Balmer series lies in the visible region.

19.

Lyman series.

20.

\(b=\frac{Z e^2 \cot \quad \theta / 2}{4 \pi \epsilon_0\left(\frac{1}{2} m v^2\right)}\)

Generic 5

2 x 5 = 10

21.

(i) The total energy of electron in ground state (n = 1) is
\({ E }_{ 1 }=-\cfrac { 13.6 }{ { 1 }^{ 2 } } eV=-13.6eV\)
It indicates that 13.6eV energy is needed to remove an electron from hydrogen atom in its ground state.
(ii) Energy in first excited state(n = 2) is
\({ E }_{ 2 }=-\cfrac { 13.6 }{ { 2 }^{ 2 } } eV=-3.4eV\)
It implies that energy needed to remove an electron from hydrogen atom in first excited state is 3.4eV, which is less than 13.6eV.

22.

(i) Nuclear reactors are used in electric power generation. They are also used to produce radioactive isotopes which have applications in medicine, industry and agriculture.
(ii) Radioactive waste consists of fission products and transuranic elements such as plutonium and americium. This waste is extremely hazardous to all forms of life on earth.
(iii) People often oppose the location site of a nuclear reactor because any leakage of nuclear radiations can affect adversely miles of area surrounding it. Elaborate safety measures are needed not only for reactor operations, but also for handling and disposal of radioactive waste.
I would suggest that Government must take stringent safety measures and assure people of safeguards in the event of nuclear accidents. At the same time, people must also be educated accordingly.