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Quantum Mechanics
& Spectroscopy
Atomic
Spectra
Absor
ption
Spectr
a
Emi
ssio
n
spe
ctra
Atomic
Structure
Unused
Sectio
n
Space
1
Rutherford
Model
J J
Thomson
Model
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The Smallest Indivisible Part Of
A Substance Is Called As
AN ATOM
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Structure of the Atom
Evidence in 1900 indicated thatthe atom was not a fundamental unit:
1) There seemed to be too many kinds
of atoms, each belonging to a distinct chemicalelement (way more than earth, air, water, and
fire!)
2) Elements combine with some elements but not
with others, a characteristic that hinted at aninternal atomic structure.
3) The discoveries of radioactivity, x rays, and the
electron all seemed to involve atoms breaking
apart in some way
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Electrons (discovered in
1897) carried the negative
charge.
Electrons were very light,
even compared to the atom.
Protons had not yet been
discovered, but clearly
positive charge had to be
present to achieve chargeneutrality.
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Thomsons plum-pudding model of the atom had thepositive charge spread uniformly throughout a sphere the
size of the atom, with electrons embedded in the uniform
background.
In Thomsons view, when the atom was heated, the electrons
could vibrate about their equilibrium positions, thus producing
electromagnetic radiation.
Unfortunately, Thomson couldnt explain spectra with thismodel.
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Experiments of Geiger and Marsden
Rutherford, Geiger, and Marsdenconceived a new technique forinvestigating the structure ofmatter by scattering a particlesfrom atoms.
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Rutherford Explanation & Model
Atom is composed ofTiny nucleus in which its
positive charge and
nearly all mass was
concentrated, with
electrons placed some
distance away.
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R
Fc = Fe
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Rutherford Scattering Formula
N() = Ni n t Z 2e482 r2KE2sin4(/2)Gold Nucleus Radius= 3.0 x 10-14 m
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Splitting of Wavelengths
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Absorption Spectrum
When white light is passed through a
a gas ,the gas is found to absorb light
of certain wavelength .So we get a
bright spectrum with dark lines on it.This is known as Absorption
spectrum.
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It is observed that for every line in atomic absorption
spectrum of an element , there is a line in emission
spectrum. However reverse is not true.
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Emission Spectrum
When an atomic gas or vapor at small
pressure is suitably excited , the
emitted radiation has a spectrum
which contains certain specificwavelengths only. Such a spectrum is
known as Emission Spectrum.
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Hydrogen spectrum
Lyman UV R(1/12-1/n2)
Balmer Visible R(1/22-1/n2)
Paschen IR R(1/32-1/n2)
Brackett IR R(1/42-1/n2)
Pfund IR R(1/52-1/n2)
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Bohrs postulates
1)An electron moves in a circular orbit around the nucleus underinfluence of columbic force.
2) It is only possible for electron to move in an orbit for which its
orbital angular momentum L is integral multiple of h/2 .L=nh/2
3)Despite the fact that electron is constantly accelarating,it will not
radiate.
4)EM wave will be radiated when electron jumps from one orbit to
other.