Identification of Isotopes
Identification of Isotopes
** The positive
rays produced in a discharge tube consist of nuclei of atoms.
** The
deflection of positive rays in an electric and magnetic field is proportional
to e/m, the charge on the particle divided by its mass.
** The nuclei
obtained from an element consisting of a mixture of isotopes will have the same
positive charge and, therefore, their deflection will be inversely proportional
to their masses. Thus with a suitable application of electric and magnetic
field, we can identify the isotopes present in a given element.
(1) Aston’s Mass Spectrograph
** In 1919 F.W.
Aston developed an instrument known as the Mass Spectrograph which can
accurately sort out the positive ions of different isotopes of an element and
determine their masses.
In this
apparatus:
(1) a beam of
positive rays obtained from a gaseous element in the discharge tube, is
rendered into a fine ribbon by passing through slits S1 and S2.
(2) The fine
beam consisting of positive ions of the various isotopes of the element is then
sent between the electrostatically charged plates P1 and P2.
(3) Here the
beam of positive ions is deflected down toward the negative plate.
(4) The slow
moving ions of the same isotope are deflected more than the faster ones which
causes a broadening of the beam.
(5) Also, the
heavier particles are deflected more (being slower) than the lighter ones and
this brings about a separation of the various isotopes.
(6) The
broadened beam of ions is then subjected to a magnetic field (shown by dashed
circle) at right angles to the plane of the charged plates and is thus sent in
a direction opposite to that caused by the electrostatic field, slower
particles again being deflected most.
(7) By
adjustment of the two fields all ions of the same mass come to focus on the
same point on the photographic plate where a sharp line is obtained.
(8) Thus each
line recorded on the photographic plate shows the existence of separate
isotope. Further, the intensity of the line in comparison with the lines of
other isotopes, gives the relative abundance of this particular isotope.
** The mass of a
particle corresponding to a line produced on the photographic plate is
determined by comparing with a standard line produced by a particle of known
mass (say, O+ = 16).
** For example,
the examination of a sample of neon and chlorine by Aston’s Mass Spectrograph
showed that they were made of Ne-20, Ne-22 and Cl-35, Cl-37 respectively. The
intensities of their lines showed that the relative abundance was:
** Thus Aston’s
Mass spectrograph not only helped in identifying the isotopes present in an
element but also helped in determining the average atomic mass of a given
element.
Example: A Sample of neon is found to consist of 20Ne, 21Ne
and 22Ne in the following percentages :
Calculate the
atomic mass of neon?
Solution:
The atomic mass of an ordinary isotopic mixture
is the average of the determined atomic masses of individual isotopes. Thus :
20 × 0.9092 =
18.18
21 × 0.0026 =
0.055
22 × 0.0882 =
1.94
=
20.18
The atomic mass
of neon is 20.18
(2) Dempster’s Mass Spectrograph
In this
apparatus:
(1) a slow stream of gas or
vapour of the sample under examination is passed in between two perforated
plates.
(2) Here it is bombarded by
high-energy electrons shot out from an electron gun.
(3) The gas atoms are thus
stripped of an electron and are converted to mono-positive ions (atom – e =
ion 1+).
(4) When a potential of 500
to 2000 volts is applied between the perforated accelerating plates, the
positive ions are strongly attracted to the negative plate.
(5) The beam of positive
ions moving with accelerated speed then enters the magnetic field at right
angles to its path and is thus made to move in a circular path.
** If V is the potential
applied across the accelerating plates and e the charge on each positive
particle, the electrical energy is Ve. This is imparted to the particles as
kinetic energy, 1/2 mv2. Thus,
** In the magnetic field of
strength H, the magnetic force on the particle Hev, exactly balances the
centrifugal force, mv2/r, r being the radius of the circular path.
Thus,
Eliminating ν between (1)
and (2), we have
e = the unit electrical
charge, and r (depending on particular apparatus) are constant. If during an
experiment magnetic field H is kept the same, from the last equation it follows
that
** Thus by adjusting the
accelerating potential (V), particles of mass m can be made to fall on the
collector plate. Each ion sets up a minute electric current which passes to the
electrometer.
** The
strength of the current thus measured, gives the relative abundance of the
particles of mass m.
** Similarly, the particles
of the other isotopes having different masses are made to fall on the collector
and current strength measured.
** The current strength in
each case gives the relative abundance of the individual isotopes.
** By comparing the current
strengths with an experiment performed with C-12 ion, the mass numbers of the
various isotopes can be determined.
** In the modern mass
spectrographs, each ion strikes a detector, the ion current is amplified and
fed to a recorder. The recorder makes a graph showing relative abundance
plotted against mass number.
** A computer-plotted graph
of neon isotopes is shown in the following Figure:
Reference: Essentials of Physical Chemistry /Arun
Bahl, B.S Bahl and G.D. Tuli / multicolour edition. .
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