Structure of Atom 2 Flashcards
what were the problems to finding out
- to account for the stability of atom after the discovery
of sub-atomic particles, - to compare the behaviour of one element from other
in terms of both physical and chemical properties, - to explain the formation of different kinds
of molecules by the combination of
different atoms and, - to understand the origin and nature of the
characteristics of electromagnetic
radiation absorbed or emitted by atoms
what is jj thomsons model of the aotm
J. J. Thomson, in 1898, proposed that an
atom possesses a spherical shape (radius
approximately 10–10 m) in which the positive
charge is uniformly distributed. The electrons
are embedded into it in such a manner as to
give the most stable electrostatic arrangement
what was the main charac of jj thomson modle
An important
feature of this model is that the mass of the
atom is assumed to be uniformly distributed
over the atom. Although this model was able
to explain the overall neutrality of the atom,
but was not consistent with the results of later
experiments.
what is rutherford alpha particle scattering expt
A stream of high
energy α–particles from a radioactive source
was directed at a thin foil (thickness ∼ 100
nm) of gold metal. The thin gold foil had a
circular fluorescent zinc sulphide screen
around it. Whenever α–particles struck the
screen, a tiny flash of light was produced at
that point.
what were obs of gold scattering
(i) most of the α– particles passed through
the gold foil undeflected.
(ii) a small fraction of the α–particles was
deflected by small angles.
(iii) a very few α– particles (∼1 in 20,000)
bounced back, that is, were deflected by
nearly 180°
.
what are the inferences of gold scattering
(i) Most of the space in the atom is empty
as most of the α–particles passed
through the foil undeflected.
(ii) A few positively charged α– particles were
deflected. The deflection must be due to
enormous repulsive force showing that
the positive charge of the atom is not
spread throughout the atom as Thomson
had presumed. The positive charge has
to be concentrated in a very small volume
that repelled and deflected the positively
charged α– particles.
(iii) Calculations by Rutherford showed that
the volume occupied by the nucleus is
negligibly small as compared to the total
volume of the atom. The radius of the
atom is about 10–10 m, while that of
nucleus is 10–15 m.
enrst reutherford model of aotm
The positive charge and most of the mass
of the atom was densely concentrated
in extremely small region. This very small
portion of the atom was called nucleus
by Rutherford.
(ii) The nucleus is surrounded by electrons
that move around the nucleus with a
very high speed in circular paths called
orbits. Thus, Rutherford’s model of atom
resembles the solar system in which the
nucleus plays the role of sun and the
electrons that of revolving planets.
(iii) Electrons and the nucleus are held
together by electrostatic forces of
attraction.
drawback of rutherford model of atom
When a body is moving in a circular path, it undergoes acceleration even if its moving with constant velocity coz direction keep changing.
acc to rutherford model, the electrons move in circular paths around circular.
according to maxwell, charged
particles when accelerated should emit
electromagnetic radiation
Therefore, an electron in an orbit will continuoulsy lose enrgy and fall into the nucleus. it will take 10 power -8 secs to do this, but this is not tru, because atom is stable.
Rutherford model is that it says nothing
about the electronic structure of atoms i.e.,
how the electrons are distributed around the
nucleus and what are the energies of these
electrons.
if the e- was stationary then it would eventaully be attracted by nucleus and hence fall into it and end up being a miniature thomsom model.
