# Atoms by nehalwan

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20                                                   Science and Technology Workbook for Class X

Chapter-12
ATOMS
Q.1 You are given two nuclei 3 X 7 and 3 Y4 .
i)  Are they isotopes of the same element, why?
ii) Which of the two is likely to be more stable, why?                                    (2 marks)
1.  i)  Yes, because both the nuclei have the same atomic number.
ii) X is likely to be more stable than Y because,
The neutron proton ratio is closer to unity in X than in Y.
The number of neutron is more than the number of protons in X.
Q.2 What is the distance of closest approach when a 5.0 MeV proton approaches a gold nucleus (Z=79)?
(2 marks)

2KZe2 1
2.   P.E =            = mv o2
ro  2

2KZe2     2 × 9 ×109 ×79 × (1.6 × −19 ) 2
10
r0      =            =                     −13
 mv 2            5 × 1.6 ×10
o
      
 2 

As (1 Mev = 1.6 × 10–13 J)
∴ ro = 45.5 × 10–15 = 45.5 fm
Q.3 Obtain approximately the ratio of nuclear radii of the gold isotope 79 Au197 and the silver isotope 47 Ag107 .
What is the approximate ratio of their nuclear densities.                                         (2 marks)
3.    R = R 0 A1 / 3

1

R(Au)  197 3
We have      =   = 1.23
R(Ag)  107
The nuclear densities remains constant for all nuclei.

ρAu
∴       ρAg    =1
Q.4 The ionisation energy of Hydrogen atom is 13.6 eV. Following Bohr’s theory , what is the energy
corresponding to a transition between the 3rd and the 4th orbit?                                (2 marks)
4.  En = 13.6/n2 , E3 = –(13.6/9) = –1.51 eV and E4 = – (13.6/16) = –0.85 eV
Therefore, E4 – E3 = –0.85 – (–1.51) = 0.66 eV
Q.5 Ionization potential of hydrogen atom is 13.6 eV. Hydrogen atom in the ground state are excited by
monochromatic radiation of photon energy 12.1 eV. How many spectral lines are emitted by hydrogen
atoms according to Bohr’s theory?                                                               (2 marks)
5.  Given that         (I.E.)H = –13.6 eV
After absorbing 12.1 eV energy, the energy becomes –13.6 eV + 12.1 eV = –1.5 eV.
This energy corresponds to n = 3 level i.e., hydrogen atoms are excited to n = 3 level.
Hence following three transitions are possible
n = 3 to n = 1, n = 3 to n = 2, and n = 2 to n = 1. Therefore three spectral lines are emitted.

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