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Unit: 8 Atomic Nucleous Batch: +2Q1. Describe the Rutherford’s α-rays scattering experiment. What were his observations and
what explanations (results) he suggested on the basis of his observations?
Ans. Ruther ford Model of atom (Experiment): Ruther ford performed experiment with the help of ' α'
particle incidence on gold foil and observed their scattering on screen with the help of detector,
Observations are
belector
Screen
Slit 9 old foil
(i) Most of ' α' particles goes undeviated.
(ii) Few ' α' particles suffers large deflection.
(iii) Very few α particles retrace their path ie recom at 180 o.
The graph btw. No. of α particles scattered at angle θ ie N (θ) and ' θ' is show in fig.
y
Ruther ford model of atom (Results Explanation): N(θ)
(i) Most of the charge and almost all the mass is concentrated at
− 10 −15 m .
centre of atom called as nucleous, Size of nucleous ~ x
60 120 180
θ
(ii) Electron revolves around the nucleous. The centriphetal force to the electron is provided by
electrostatic force of attraction btw. electron & nucleous
(iii) Electrons revolve around the nucleous are not stationary. If they are stationary, then they will
fall in the nucleous
(iv) Size of atom is of the order of 10 −10 m
Q2. Write the draw backs of Rutherford model of an atom.
Ans. Draw backs of Rutherford model of atom:
(i) It could not explain the stability of atom: according to Rutherford electron revolve around the
nucleous and have centriphetal acc. hence radiate energy in the form of e.m. radiations which
decrease the energy of revolving electron. Hence radius of circular path decreses and finally
electron will fall in nucleous.
(ii) It could not explain the spectrum of atom: Acc. to Rutherford electron revolve around the
nucleous in all the orbits. Hence emit radiations of all wave lengths that mean spectrum of
hydrogen should be a continuous spectrum, but it is a line spectrum.
1
,Q1. Describe the Rutherford’s α-rays scattering experiment. What were his observations and
what explanations (results) he suggested on the basis of his observations?
Ans.
Q2. Write the draw backs of Rutherford model of an atom.
Ans.
2
, Q3. State the basic Postulates of Bohr’s theory of atomic spectra. Hence obtain an expression for
radius of orbit and velocity of electron in Bohr’s stationary orbit.
Ans. Bohr’s model of Hydrogen atom:
(i) Every atom consists of central core called nucleous, in which entire + ve charge and almost
entire mass of atom are concentrated. Electrons revolve around nucleus in circular orbits. The
centriphetal force required for revolution is provided by electrostatic force of attraction btw.
electron and nucleus.
mv 2 1 mv 2 kze 2
i.e. = Ze × e Or = 2
r 4π ∈o r r
1
Where m is mass of electron ‘e’ is charge on electron. Z is atomic no. and k =
4π ∈o
(ii) Electro can revolve in only those stationary orbits whose total angular momentum is a integral
nh
multiple of h / 2π where ‘h’ is plank’s const. i.e. mvr = . ………..(1)
2π
(iii) Emission or absorption of energy takes place when electron jumps from one orbit to another
orbit.
If E1 is the energy in inner orbit and E 2 is the energy in outer orbit then energy released when
electron jump from outer orbit to inner orbit E 2 − E1 = hv ………..(3)
‘ν’ – freq. of radiations emitted.
(a) Radii of Boht’s stationary orbits
nh
From (2) mvr =
2π
nh
v= (put in eqn. 1)
2πmr
m n 2h 2 kZe 2
=
r 4π 2m 2r 2 r2
n 2h 2 n2
Or r= i.e. r ∝ ………..(4)
4π 2 mke 2 Z
For hydrogen atom Z = 1
i.e. r ∝ n2 i.e. radii of stationary orbits is in ratio of 1: 4: 9.
i.e. stationary orbits are not equally spaced. Substitute various values in (4).
For n = 1, r = 0.529 A o
kze 2 nh
(b) Velocity of electron in Bohr’s Stationary orbit: From (1) and (2), r = and r =
mv 2
2πmv
kze 2 nh 2π kze 2 Z
∴ = V= V∝
mv 2
2πmv nh n
2π ke 2
For hydrogen atom Z = 1, V= ………..(5)
nh
For first orbit n = 1 hence v= 1/137 times velocity of light.
3
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