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Chapter: 12th Physics : UNIT 9 : Atomic and Nuclear Physics

Short answer questions

Physics : Atomic and Nuclear Physics: Book Back Important Questions, Answers, Solutions: Short answer questions

II Short answer questions


1. What are cathode rays?.

Cathode rays are streams of electrons emitting from cathode in a Coolidge tube at a pressure of around 0.01 mm of Hg.

2. Write the properties of cathode rays.

1) Cathode rays possess energy and momentum

2) They travel in a straight line

3) They are deflected by electric and magnetic fields

4) They affect photographic plates

5) They produce fluorescence when they fall on certain crystals and minerals

6) They ionize the gas through which they pass

7) They produce heat, when they fall on matter

8) They are negatively charged particles nothing but electrons

9) When the cathode rays fall on a material of high atomic weight, X-rays are produced.

10) The speed of cathode rays is upto (1/10)th  of the speed of light.

3. Give the results of Rutherford alpha scattering experiment.

1) Most of the alpha particles are undeflected through the gold foil and went straight

2) Some alpha particles are deflected through a small angle

3) A few alpha particles are deflected through the angle more that 900.

4) Very few alpha particles returned back ie., deflected back by 180°.

4. Write down the postulates of Bohr atom model.

1) The electron in an atom moves around nucleus in circular orbits under the influence of Coulomb electrostatic force of attraction. This force gives necessary centripetal force for the electron to undergo circular motion.

2) Electrons in an atom revolve around the nucleus only in certain discrete orbits which does not readiate electromagnetic energy called stationary orbits. The angular momentum of the electrons in these stable orbits are quantized ie., angular momentum equal to integral multiple of h/2π

 l  = nh / 2π

This is Angular momentum Quantization condition

3) Energy of orbits are not continuous but only discrete ie, Quantization of energy.

4) An electron can jump from one orbit to another orbit by absorbing or emitting a photon whose energy is equal to the difference in energy (∆E) between the two orbital levels.

ΔE = Efinal – Einitial = hυ = hc / λ

 υ – Frequency; h-Planck’s constant

5. What is meant by excitation energy.

The energy required to excite an electron from lower energy state to any higher energy state is known as excitation energy.

6. Define the ionization energy and ionization potential.

The minimum energy required to remove an electron from an atom in the ground state is known as binding energy or ionization energy.

Ionization potential is defined as ionization energy per unit charge

7. Write down the draw backs of Bohr atom model.

The following are the drawbacks of Bohr atom model

1) Bohr atom model is valid only for hydrogen atom or hydrogen like-atoms but not for complex atoms.

2) When the spectral lines are closely examined, individual lines of hydrogen spectrum is accompanied by a number of faint lines. These are often called fine structure. This can not explained by Bohr atom model.

3) Bohr atom model fails to explain the intensity variations in the spectral lines.

4) The distribution of electrons in atoms is not completely explained by Bohr atom model.

8. What is distance of closest approach?

The minimum distance or contact distance between the centre of the nucleus and the alpha particle just before it gets reflected back through 180° is defined as the distance of closest approach (contact distance).

 r0 =  1/4πεo [ 2ze2/Ek ]

9. Define impact parameter.

The impact parameter is defined as the perpendicular distance between the centre of the gold nucleus and the direction of velocity vector of alpha particle when it is at a large distance.

b = Kcot (θ/2 )

where K = 1/ 4πε 0  [ 2Ze2 / mv20 ]

10. Write a general notation of nucleus of element X. What each term denotes?

General notation of nucleus of element AZX

X - Chemical symbol of the element

A - Mass number

Z - Atomic Number

N - Number of neutrons (N = A-Z)

11. What is isotope? Give an example.

Isotopes are atoms of the same element having same atomic number but different mass number.

Ex : 11H - Hydrogen, - 21H - Deuterium,

 31H - Tritium and 116C, 126C, 136C, 146C

12. What is isotone? Give an example.

Isotones are the atoms of the different elements having same number of neutrons.

Ex : 125B – Boron, 136C – Carbon. Both have 7 neutrons

13. What is isobar? Give an example.

Isobars are the atoms of different elements having the same mass number but different atomic number.

Ex: 4018Ar – Argon, 4020Ca - Calcium

14. Define atomic mass unit u.

One atomic mass unit is defined as the  1/12th  of the mass of the isotope of carbon 126C.

1 u = 1.66 x 10-27 kg

15. Show that nuclear density is almost constant for nuclei with Z > 10.

Nuclear density is defined as the ratio of the mass of the nuclei to the volume of the nuclei.

It shows that density is independent of mass number. In other words all the nuclei (z >10) have the same density.

ρ = 2.3 x 1017 kg m-3

16. What is mass defect?

The difference in the total mass of the nucleons and the actual mass of the nucleus is known as the mass defect.

Δm = (Zmp + Nmn) - M 

M - Actual mass of the nucleus

17. What is binding energy of a nucleus? Give its expression.

When the protons and neutrons combine to form a nucleus, the mass that disappears is converted into an equivalent amount of energy. This energy is called the binding energy of the nucleus.

BE = (Zmp + Nmn - MA)c2

= Δmc2

18. Calculate the energy equivalent of 1 atomic mass unit.

1u = 1.67 × 10-27 kg

Energy equivalent of lu = u × c2

= 1.66 × 10-27 x ( 3× l08 )2

= 1.66 × l0-27 × 9 × 1016

= 14.94 × 10-11 J

= 14.94 × 10-11 /1.6 × 10-19   eV

= 931 × 106 eV = 931 MeV

Energy equivalent of l u = 931 MeV

19. Give the physical meaning of binding energy per nucleon.

The average binding energy per nucleon is the energy required to separate a single nucleon from the particular nucleus.

20. What is meant by radioactivity?

The phenomenon of spontaneous emission of highly penetrating radiations such as α,β and γ rays by an element is called radioactivity and the substances which emit these radiations are called radioactive elements.

21. Give the symbolic representation of alpha decay, beta decay and gamma decay.

22. In alpha decay, why the unstable nucleus emits 42He nucleus? Why it does not emit four separate nucleons?

42He consists of two protons and two neutrons. For example if 23892U nucleus decays into 23490Th by emitting four separate nucleons (two protons and two neutrons), then the disintegration energy Q is negative.

It implies that the total mass of products is greater than that of parent (23892U) nucleus.

This kind of process cannot occur in nature because it would violate conservation of energy. In any decay process

conservation of energy

conservation of linear momentum

conservation of angular momentum must be obeyed. So, the unstable nucleus emits 42He nucleus.

23. What is mean life of radioactive nucleus? Give the expression.

The mean life time of the nucleus is the ratio of sum of life times of all nuclei to the total number of nuclei present initially

τ = 1/ λ =1.443 T 1 /2

Mean life time is also equal to reciprocal of decay constant.

24. What is half-life of nucleus? Give the expression.

The half life (T1/2) is defined as the time required for the number of atoms initially present to reduce to one half of the initial amount.

T1/2= 0.6931/ λ

25. What is meant by activity or decay rate? Give its unit.

Activity is defined as the number of nuclei decayed per second.


SI unit of activity is Becquerel (Bq) and standard unit is Curie (Ci).

26. Define curie.

One curie is defined as the number of decays per second in lg of radium and it is equal to

3.7 x 1010 decays/s.

1 Ci = 3.7 x 1010 Bq


27. What are the constituent particles of neutron and proton?

Neutron consists of one up quark and two down quarks.

Proton consists of one down quark and two up quarks.

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