PROVING THE PRESENCE OF ISOTOPES

By Rania, Cheryl, ZhenJie and Ivy

Definition of Isotopes

 Isotopes are two or more of the Isotopes are two or more atoms that have the same number of protons but different number of neutrons.

  different atomic mass (mass number).

In other words, they have the same atomic numbers (Z) But different mass numbers (A).

numbers of neutrons.

Examples of Isotopes

Neutrons

Similarities?

Nucleus Carbon

Differences?

Protons

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 mass to-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

    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, 9 th July 2008 The Cambridge Dictionary of Scientists 5 th July 2008 - Last accessed, http://en.wikipedia.org/wiki 2008 - Last accessed, 9 th July http://www-outreach.phy.cam.ac.uk/camphy /positiverays/positiverays_index.htm

- Last accessed, 17 th July 2008