CBSE Class 12 Physics Atoms And Nuclei Worksheet

 

Question. According to the Bohr theory of H-atom, the speed of the electron, its energy and the radius of its orbit varies with the principal quantum number n, respectively, as 

Answer: D

Question. Which of the following series in the spectrum of hydrogen atom lies in the visible region of the electromagnetic spectrum?
(a) Paschen series
(b) Balmer series
(c) Lyman series
(d) Brackett series

Answer: B

Question. The following statements are all true. Which one did Rutherford consider to be supported by the results of experiments in which a-particles were scattered by gold foil?
(a) The nucleus of an atom is held together by forces which are much stronger than electrical or gravitational forces.
(b) The force of repulsion between an atomic nucleus and an a-particle varies with distance according to inverse square law.
(c) α-particles are nuclei of Helium atoms.
(d) Atoms can exist with a series of discrete energy levels.

Answer: B

Question. In a hypothetical Bohr hydrogen atom, the mass of the electron is doubled. The energy E₀' and radius r₀' of the first orbit will be (r₀ is the Bohr radius)
(a) –11.2 eV
(b) –6.8 eV
(c) –13.6 eV
(d) –27.2 eV

Answer: D

Question. A 15.0 eV photon collides with and ionizes a hydrogen atom. If the atom was originally in the ground state (ionization potential =13.6 eV), what is the kinetic energy of the ejected electron?
(a) 1.4 eV
(b) 13.6 eV
(c) 15.0 eV
(d) 28.6 eV

Answer: A

Question. The ratio of areas between the electron orbits for the first excited state to the ground state for the hydrogen atom is
(a) 2 : 1
(b) 4 : 1
(c) 8 : 1
(d) 16 : 1

Answer: D

Question. The potential energy associated with an electron in the orbit
(a) increases with the increases in radii of the orbit
(b) decreases with the increase in the radii of the orbit
(c) remains the same with the change in the radii of the orbit
(d) None of these

Answer: B

Question. Energy levels, A, B, C of a certain atom correspond to increasing values of energy i.e. E_A < E_B < E_C. If λ₁, λ₂, λ₃ are the wavelengths of radiations corresponding to the transitions C to B, B to A and C to A respectively, which of the following relations is correct ?
(a) λ₃ = λ₁ + λ₂
(b) λ₃ = λ₁λ₂/λ₁ + λ₂
(c) λ₁ + λ₂ + λ₃ = 0
(d) λ₃3 = λ₁3 + λ₂²

Answer: B

Question. As the quantum number increases, the difference of energy between consecutive energy levels
(a) remain the same
(b) increases
(c) decreases
(d) sometimes increases and sometimes decreases.

Answer: C

Question. In Bohr’s model of the hydrogen atom, let R, V, T and E represent the radius of the orbit, speed of the electron, time period of revolution of electron and the total energy of the electron respectively. The quantity proportional to the quantum number n is
(a) VR
(b) E
(c) r
(d) T

Answer: A

Question. Let ν₁ be the frequency of the series limit of the Lyman series, ν₂ be the frequency of the first line of the Lyman series, and ν₃ be the frequency of the series limit of the Balmer series.
(a) ν₁ – ν₂ = ν₃
(b) ν₂ – ν₁ = ν₃
(c) ν₃ = 1/2(ν₁+ν₂)
(d) ν₁ + ν₂ = ν3

Answer: A 

Question. Excitation energy of a hydrogen like ion in its excitation state is 40.8 eV. Energy needed to remove the electron from the ion in ground state is
(a) 54.4 eV
(b) 13.6 eV
(c) 40.8 eV
(d) 27.2 eV

Answer: A

Question. The approximate value of quantum number n for the circular orbit of hydrogen 0.0001 mm in diameter is
(a) 1000
(b) 60
(c) 10000
(d) 31

Answer: D

Question. The proof of quantization of energy states in an atom is obtained by the experiment performed by
(a) Thomson
(b) Millikan
(c) Rutherford
(d) Franck and Hertz

Answer: D

Question. The Rutherford a-particle experiment shows that most of the a-particles pass through almost unscattered while some are scattered through large angles. What information does it give about the structure of the atom?
(a) Atom is hollow.
(b) The whole mass of the atom is concentrated in a small centre called nucleus
(c) Nucleus is positively charged
(d) All the above

Answer: D

Question. The ratio of maximum to minimum wavelength in Balmer series is
(a) 3 : 4
(b) 1 : 4
(c) 5 : 36
(d) 5 : 9

Answer: D

Question. If the frequency of K_α X-rays emitted from the element with atomic number 31 is n, then the frequency of K_α X-rays emitted from the element with atomic number 51 would be
(a) 5/3 ν
(b) 51/31 ν
(c) 25/9 ν
(d) 9/25 ν

Answer: C

Question. What element has k_α line of wavelength 1.785 Å?
R = 109737 cm^-1.
(a) Platinum
(b) Zinc
(c) Iron
(d) Cobalt

Answer: D

Question. If the k_α radiation of Mo (Z = 42) has a wavelength of 0.71Å. Calculate the wavelength of the corresponding radiation of Cu (Z = 29).
(a) 1.52Å
(b) 2.52Å
(c) 0.52Å
(d) 4.52Å

Answer: A

Question. Line spectrum is obtained whenever the incandescent vapours at low pressure of the excited substance are in their
(a) atomic state
(b) molecular state
(c) nuclear state
(d) None of these

Answer: A

Question. The extreme wavelengths of Paschen series are
(a) 0.365 mm and 0.565 mm
(b) 0.818 mm and 1.89 mm
(c) 1.45 mm and 4.04 mm
(d) 2.27 mm and 7.43 mm

Answer: B

Question. The third line of Balmer series of an ion equivalnet to hydrogen atom has wavelength of 108.5 nm. The ground state energy of an electron of this ion will be
(a) 3.4 eV
(b) 13.6 eV
(c) 54.4 eV
(d) 122.4 eV

Answer: C

Question. Given the value of Rydberg constant is 10⁷m^–1, the wave number of the last line of the Balmer series in hydrogen spectrum will be : 
(a) 0.025 × 10⁴ m^–1
(b) 0.5 × 10⁷ m^–1
(c) 0.25 × 10⁷ m^–1
(d) 2.5 × 10⁷ m^–1

Answer: C

Question. Electrons in the atom are held to the nucleus by
(a) coulomb’s force
(b) nuclear force
(c) vander waal’s force
(d) gravitational force

Answer: A

Question. In Millikan’s oil drop experiment, an oil drop is observed to move vertically upward. The upward motion of the drop is due to
(a) gravity
(b) viscosity
(c) buoyancy
(d) electric field

Answer: D

Question. In a Rutherford experiment, the number of particles scattered at 90° angle are 28 per minute then number of scattered particles at an angle 60° and 120° will be
(a) 117 per minute, 25 per minute
(b) 50 per minute, 12.5 per minute
(c) 100 per minute, 200 per minute
(d) 112 per minute, 12.4 per minute

Answer: D

Question. In Hydrogen spectrum, the wavelength of H_α line is 656 nm, whereas in the spectrum of a distant galaxy, H_α line wavelength is 706 nm. Estimated speed of the galaxy with respect to earth is
(a) 2 × 10⁸ m/s
(b) 2 × 10⁷m/s
(c) 2 × 10⁶ m/s
(d) 2 × 10⁵ m/s

Answer: B

Question. The angular speed of the electron in the nth orbit of Bohr hydrogen atom is
(a) directly proportional to n
(b) inversely proportional to √n
(c) inversely proportional to n²
(d) inversely proportional to n3

Answer: D

Question. The angular momentum of electron in n^th orbit is given by
(a) nh
(b) h/2πn
(c) n(h)/2π
(d) n²(h)/2π

Answer: C

One Marks Question

Question. Why do α particles have high ionizingpower?

Answer : because of their large mass & large nuclear cross section

Question. Write the relationship between the half-life& the average life of a radioactive substance?
Answer : T =1.44t1/2

Question. Name the physical quantity whose dimensions are same as Planck'sconstant.
Answer : Angular momentum

Question. Draw graph between no. of nuclei un-decayed with time for a radioactive substance.
Answer : 

Question. What is the neutron multiplication factor? For what value of these a nuclear reactor is said to be critical.
Answer : K=1

Question. Why fusion reactions is also known as thermos nuclear reactions.
Answer : Fusion need very huge temperature of the order of 10⁶-10⁷ K.

 

Two Marks questions:

Question. What is mass defect.
Answer : It is found that mass of stable nucleus is always less than the sum of the masses of its constituents neutrons and protons in their free state

Question. Distinguish between isotopes and isobars and isotones
Answer : Same atomic number but different mass number, same mass number different atomic number, same number of neutrons. 

Question. What is packing fraction.What is its physical significance?
Answer : Ratio of mass defect and mass number, PF is related to the stability of the nucleus.

Question. What is the nuclear holocaust?
Answer : It is a dangerous situation of releasing uncontrolled huge energy from nuclear reaction.

Question. What is nuclear forces, give it important properties ?
Answer : Forces among the nucleons of a nucleus is called nuclear forces. Properties:- strong, short range, charge independent, non central.

THREE MARKS QUESTION:

Question. Write limitations of Rutherford model of atom.
Answer : It fails to explain the stability of an atom, it fails to explain the line spectrum of an atom

Question. Give one similarity and dissimilarity between nuclear fission and nuclear fusion .
Answer : Similarity- both are the source of enormous amount of energy, dissimilarity- in fission heavy nuclei splits and in fusion lighter nuclei combines

Question. Draw a graph showing the variation in the binding energy per nucleons with mass number of different nuclei. Give some salient features of the curve.
Answer : 

• Binding energy per nucleon is small for lighter nuclei
• Heavier nuclei is unstable
• Fe has the highest binding energy so no nuclear reaction

Question. Define binding energy and biding energy per nucleons.
Answer :  Energy required to break the nucleus
Energy required to extract one nucleon from nucleus
Binding energy per nucleon = (Binding energy)/(mass number)

 

1 How do the neutron to Proton Ratio change during

i) β+   ii) β-   iii) α emission?

 

2 Why it is difficult to detect to neutrinos and anti neutrinos ?

 

3 Define reproduction factor or multiplication factor or k factor ? 

 

4 State displacement law for alpha decay . 

 

5 What is the role of control rods in a nuclear reactor? Name the materials used in control rods.

 

6 State the limitations of Rutherford ‘s model. 

 

7 State Bohr’s quantization principle and frequency condition.

 

8 Define impact parameter and angle of scattering and give the relation between them. 

 

9 What is enriched Uranium ? 

 

10 What is the nuclear radius of 26Fe125 if that of 13Al27 is 3.6 F?

 

11 Derive expression for the energy of electron in the nth orbits. 

 

12 Define the distance of closest approach of an alpha particle to a gold nucleus .

 

13 What is the role of a moderator in a nuclear reactor ? Explain the use of moderator in a nuclear reactor?

 

 

1 Calculate the binding energy and binding energy per nucleon of 26Fe56 nucleus. Given,mass of 26Fe56 nucleus = 55.9349 amu mass of 1 proton = 1.007825 amu mass of 1 neutron = 1.008665 amu.

 

2 Calculate the time required for 60% of a sample of radon to undergo decay. Given T½ of radon = 3.8 days

 

3 The isotope 92U238 successively undergoes three α-decays and two β-decays. What is the resulting isotope?

 

4 If 50% of a radioactive sample decays in 5 days, how much of the original sample will be left over after 20 days?

 

5 The radioactive isotope 84Po214 undergoes a successive disintegration of two α–decays and two β-decays. Find the atomic number and mass number of the resulting isotope.

 

6 Calculate the time required for 60% of a sample of radon to undergo decay. Given T½ of radon = 3.8 days

 

7 The Rydberg constant for hydrogen is 1.097 × 107ms−1. Calculate the short and long wavelength limits of Lyman series.

 

8 An α – particle is projected with an energy of 4 MeV directly towards a gold nucleus.Calculate the distance of its closest approach. Given : atomic number of gold = 79 and atomic number of α particle = 2.

 

9 Wavelength of Balmer first line is 6563Å. Calculate the wavelength of second line.