PROVING THE PRESENCE
OF ISOTOPES
By Rania, Cheryl, ZhenJie
and Ivy
Definition of Isotopes
Isotopes are two or more of the
Isotopes
two or more
atoms
that
different are
species
of the
same
have
the same
numbereach
of protons
chemical
element
havingbut
different
of neutrons.
different number
atomic mass
(mass number).
InIsotopes
other words,
havehave
the same
of an they
element
nuclei
atomic
numbers
(Z)
with the same number of protons (the
But
different
numbers
(A).
same
atomicmass
number)
but different
numbers of neutrons.
Examples of Isotopes
Neutrons
Protons
Nucleus
Carbon
Similarities?
Differences?
Discover the Presence of Isotopes
Joseph John Thomson was
the one who discovered
isotopes. Born in 1856 in
Manchester, he studied at
Trinity College. He was also
the one who discovered the
electrons.
The discovery of the isotopes
are also related to that of the
electrons.
Discover the Presence of Isotopes
Wonder about the rays that moves
towards the cathode?
They are positively charged.
Experiments showed that these rays:
consist of massive particles and
the charge of the positive particles is the same
in magnitude as the electrons.
Discover the Presence of Isotopes
Through calculations done, it was clear
that there are groups of atoms that are:
of the same element but different mass
which is not due to the number of positively or
negatively charged particles.
We shall use Aston’s better method (than
Thomson’s) to explain what is being done.
ASTON, Francis William
 ASTON, FRANCIS
WILLIAM. (1877-1945)
British chemical
physicist: he invented
the mass spectrograph,
which could determine
the existence of isotopes
in an element.
Proving the existence of isotopes
 Using Aston’s mass
spectrograph. HOW?
 Mass spectrometry is also
used to determine the isotopic
composition of elements within
a sample.
 A very sensitive instrument as:
Differences in mass among
isotopes are very small; and
Less abundant isotopes are
very rare.
The Mass Spectrometer
Electromagnetic
force is used to
separate
different isotopes
of the same
element.
The Mass Spectrometer
Before letting the
spectrometer
determine the
isotope mass ratio,
the substance is
being turned into
gaseous form as
well as becoming
electrically
charged (ions).
The Mass Spectrometer
After the
substance is
charged, it is
being repelled
into the
spectrometer.
The Mass Spectrometer
The analyzer
(which exerts
electromagnetic
force) bends the
ray of ions, as
there is a
(positive)
charge.
The Mass Spectrometer
As some
isotopes are
heavier, they will
be repelled
more from the
magnetic force.
Hence the
resulting path of
the heavier
isotopes are
less bent.
The Mass Spectrometer
As some
isotopes are
heavier, their
speed will
become slower
than those
which are lighter
due to the
repelling force.
The Mass Spectrometer
The streams of
sorted ions are
passed from the
analyzer to the
detector, which
records the
abundance of
each ion type.
The Mass Spectrometer
The result will
be calculated
with the massto-charge ratio,
as the charge is
known.
The Mass Spectrometer
This information
is then used to
determine the
chemical
element
composition of
the original
sample, and the
isotopic
composition of
its components.
The Mass Spectrometer
Taking sodium
chloride as an
example…
What Aston Showed Us
Over 50 elements consisted of atoms of:
the same atomic number but
different relative atomic mass
however the differences are similar in pattern.
The apparent deviations of relative atomic
masses of the elements from integer
results imply the presence of isotopes.
After that…
 Shortly after Aston’s discovery, W.D. Hawkins and his
students from The University of Chicago used
fractional distillation to separate Mercury vapour into
six isotopes.
 This experiment led to a series of more discoveries
about isotopes in the following year, as they followed
in Aston’s footsteps.
References



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http://www.books.google.com.sg/books?id=wKzJTBZh20
wC&pg=PA234&dq=evidence+of+the+presence+of+iso
topes&sig=ACfU3U0UXYEf1FVrwi10AkbSfWbg_QB0OQ#
PPA234,M1 - Last accessed, 9th July 2008
The Cambridge Dictionary of Scientists - Last accessed,
5th July 2008
http://en.wikipedia.org/wiki - Last accessed, 9th July
2008
http://www-outreach.phy.cam.ac.uk/camphy
/positiverays/positiverays_index.htm - Last accessed,
17th July 2008