CHAPTER -12 ATOMS

 Among alpha ,beta and gamma radiations ,which get  affected by electric field ?

What will be ratio of two nuclei of mass numbers A1 and A2?

Compare the radii of two nuclei numbers 1 and 27 respectively?

What is the nuclear radius of ¹²⁵ Fe if that of ²⁷ Al is 3.6 fermi?

Assuming the nuclei to be spherical in shape ,how does the surface area of a nucleus of

mass number A1 compare with that of a nucleus of mass number A2?

   What is the ratio of nuclear densities of the two nuclei having mass number in the  ratio 1:4?          

    What happens to the neutron to proton ratio after the emission of α-particle?

Two nuclei have mass number in the ratio of 2: 5 . What is the ratio of  their nuclear densities ?

Which has greater ionizing particle :  α-particle  or β-particle 

What is the difference between an electron and a β-particle?

Why do  α-particle   have high ionizing power?

   A nucleon  of mass number A , has  a defect Δm .Give the formula ,for the binding energy per nucleon , of  this nucleus .

The binding energy per nucleon of the two nuclei A and B are 4 MeV and 8 -2 MeV .Which of accompanied

by release of energy ,Which of the  two  nucleus is more stable?

    Four nuclei of an element fuse together to form a heavier nucleus .If the process is accompanied by release  of energy ,which of the two the parent or the daughter nucleus would have a higher binding energy /nucleon ?

Name the reaction responsible for energy production in the sun

Write a typical nuclear reaction in which a large amount of energy is released in the process of number     Fission?

            Write any one equation representing nuclear fusion reaction?

 Give the mass number and atomic number of elements on the right hand side of the decay process?

State condition for controlled chain reaction in a nuclear reactor ?

            Name  the    absorbing material used to control the reaction rate of neutron in a  nuclear reactor?

What a percentage of a given mass of  a radioactive substance will be  undecayed  after four half periods?

Write the nuclear decay process for nuclide β-decay of  ₁₅P³² ?

Define the term activity of a  radio nuclide .Write its S.I Unit .

If the nucleon a nucleus are separated far apart from each other,the sum of masses of all these nucleons is   larger  than the mass of the nucleus .where does this  mass difference come from?

Write two characteristic features of nuclear force which distinguish it from Coulomb’s force?

Define ionization energy .What is its value for hydrogen atom?

Define half -life of a radioactive sample .which of the following  radiations : α –rays ,β –rays and γ-rays

(i)                 Are similar to X-rays          

(ii)               Are easily absorbed by matter

(iii)             Travel with the greatest speed

(iv)              Are similar in nature to cathode rays ?

Define the term ‘ Activity ‘ of a radioactive substance .State its SI unit .Give plot of activity of a radioactive

species  versus time. Two different radioactive elements with half lives T₁ and T₂ have N₁  and  N₂

(undecayed ) atoms respectively present at a given instant .Determine the ratio of their activities at this           instant.

The half –life of ₆C¹⁴ is 5700 years. What does it mean? Two radioactive nuclei X and Y initially contain an equal number of atoms .Their half lives are 1 hour and 2 hours respectively .Calculate the ratio of their rates of disintegration after two hours?

Draw the graph showing the variation of binding energy per nucleon with the mass number. What are the  main inferences from the graph? Explain with the help of this plot the release of energy in the processes  of nuclear fission and fusion .

How does  the size of nucleus depend on its mass number ? Hence explain why the density of nuclear matter in independent of the size of nucleus?

What  does the term  stand for ?  write the term of  , the time interval ‘T’ in  which half of   the original number of nuclei of this  radio nuclei would have got decayed ?

Explain with example ,whether the neutron –proton ratio  in a nucleus in increases or decreases due to     β-decay?

Whit the help of an example  how the neutron to proton ratio changes during α –decay of a nucleus ?

Distinguish between isotops and isobars. Give one example for each of the species?

Group the following six nuclides into three pairs (i) isotones (ii) isotopes(iii) isobars ?

                   ₆C¹², ₂He³ , ₈₀Hg¹⁹⁸ , ₁H³ ,₇₉Au¹⁹⁷ , ₆C¹⁴  

You are given two nuclides ₃X⁷  and ₃Y⁴

Use basic law of radioactive decay to show that radioactive nuclei follow an exponential decay law?

Derive expression for average life of a radio nuclei .Give its relationship with half life ?

What are α –particles ? In the reaction    _ZX^A          α+γ  give the atomic number and mass numbers of Y?           

Explain why is the energy distribution of β –rays continuous/

In a nuclear reactor give the function of the (i) moderator (ii) control rods (iii) coolant (iv) heavy water .

         In the   reaction       ₉₂P²³⁶                      _aQ¹⁴¹   +  ₃₆R^b  + 3( ₀n¹)

What are the values of  a and b .

Why is the heavy water used  as a moderator in a nuclear reactor ?

Why is the mass of a nucleus always less than the sum of the masses of constituents , neutrons and protons?

Obtain the first Bohr's radius and the ground state energy of a muonic hydrogen atom [i.e., an atom in which a negatively charged muon (μ^-) of mass about 207m_e orbits around a proton].

If Bohr's quantisation postulate (angular momentum = nh/2π) is a basic law of nature, it should be equally valid for the case of planetary motion also. Why then do we never speak of quantisation of orbits of planets around the sun?

The total energy of an electron in the first excited state of the hydrogen atom is about -3.4 eV.

(a) What is the kinetic energy of the electron in this state?

(b) What is the potential energy of the electron in this state?

(c) Which of the answers above would change if the choice of the zero of potential energy is changed?

Classically, an electron can be in any orbit around the nucleus of an atom. Then what determines the typical atomic size? Why is an atom not, say, thousand times bigger than its typical size? The question had greatly puzzled Bohr before he arrived at his famous model of the atom that you have learnt in the text. To simulate what he might well have done before his discovery, let us play as follows with the basic constants of nature and see if we can get a quantity with the dimensions of length that is roughly equal to the known size of an atom (~ 10^-10 m).

(a) Construct a quantity with the dimensions of length from the fundamental constants e, m_e, and c. Determine its numerical value.

(b) You will find that the length obtained in (a) is many orders of magnitude smaller than the atomic dimensions. Further, it involves c. But energies of atoms are mostly in non-relativistic domain where c is not expected to play any role. This is what may have suggested Bohr to discard c and look for 'something else' to get the right atomic size. Now, the Planck's constant h had already made its appearance elsewhere. Bohr's great insight lay in recognising that h, m_e, and e will yield the right atomic size. Construct a quantity with the dimension of length from h, m_e, and e and confirm that its numerical value has indeed the correct order of magnitude.

Obtain an expression for the frequency of radiation emitted when a hydrogen atom de-excites from level nto level (n-1). For large n, show that this frequency equals the classical frequency of revolution of the electron in the orbit.

The gravitational attraction between electron and proton in a hydrogen atom is weaker than the coulomb attraction by a factor of about 10-40. An alternative way of looking at this fact is to estimate the radius of the first Bohr orbit of a hydrogen atom if the electron and proton were bound by gravitational attraction. You will find the answer interesting.

Answer the following questions, which help you understand the difference between Thomson's model and Rutherford's model better.

(a) Is the average angle of deflection of α­-particles by a thin gold foil predicted by Thomson's model much less, about the same, or much greater than that predicted by Rutherford's model?

(b) Is the probability of backward scattering (i.e., scattering of α-particles at angles greater than 90°) predicted by Thomson's model much less, about the same, or much greater than that predicted by Rutherford's model?

(c) Keeping other factors fixed, it is found experimentally that for small thickness t, the number of α-particles scattered at moderate angles is proportional to t. What clue does this linear dependence on t provide?

(d) In which model is it completely wrong to ignore multiple scattering for the calculation of average angle of scattering of α-particles by a thin foil?

In accordance with the Bohr's model, find the quantum number that characterises the earth's revolution around the sun in an orbit of radius 1.5 x 10¹¹ m with orbital speed 3 x 10⁴ m/s. (Mass of earth = 6.0 x 10²⁴ kg.)

A 12.5 eV electron beam is used to bombard gaseous hydrogen at room temperature. What series of wavelengths will be emitted?

The radius of the innermost electron orbit of a hydrogen atom is 5.3 x 10^-11 m. What are the radii of the n = 2 and n =3 orbits?

(a) Using the Bohr's model calculate the speed of the electron in a hydrogen atom in the n = 1, 2, and 3 levels. (b) Calculate the orbital period in each of these levels.

A hydrogen atom initially in the ground level absorbs a photon, which excites it to the n = 4 level. Determine the wavelength and frequency of the photon.

The ground state energy of hydrogen atom is -13.6 eV. What are the kinetic and potential energies of the electron in this state?

A difference of 2.3 eV separates two energy levels in an atom. What is the frequency of radiation emitted when the atom makes a transition from the upper level to the lower level?

What is the shortest wavelength present in the Paschen series of spectral lines?

Suppose you are given a chance to repeat the alpha-particle scattering experiment using a thin sheet of solid hydrogen in place of the gold foil. (Hydrogen is a solid at temperatures below 14 K.) What results do you expect?

Choose the correct alternative from the clues given at the end of the each statement:

(a) The size of the atom in Thomson's model is .......... the atomic size in Rutherford's model. (much greater than/no different from/much less than.)

(b) In the ground state of .......... electrons are in stable equilibrium, while in .......... electrons always experience a net force.

(Thomson's model/ Rutherford's model.)

(c) A classical atom based on .......... is doomed to collapse.

(Thomson's model/ Rutherford's model.)

(d) An atom has a nearly continuous mass distribution in a .......... but has a highly non-uniform mass distribution in ..........

(Thomson's model/ Rutherford's model.)

(e) The positively charged part of the atom possesses most of the mass in .......... (Rutherford's model/both the models.)