PROVING THE PRESENCE OF ISOTOPES

By Rania, Cheryl, ZhenJie and Ivy

Definition of Isotopes

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

In other words, they have the same atomic numbers (Z)

But different mass numbers (A).

Isotopes are two or more of the different species of the same chemical element each having different atomic mass (mass number). Isotopes of an element have nuclei with the same number of protons (the same atomic number) but different numbers of neutrons.

Examples of Isotopes

Carbon

NeutronsNeutronsProtonsProtons

Similarities? Differences?

NucleusNucleus

Discover the Presence of IsotopesJoseph 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 IsotopesWonder about the rays that moves

towards the cathode?They are positively charged.

Experiments showed that these rays:consist of massive particles andthe charge of the positive particles is the same

in magnitude as the electrons.

Discover the Presence of IsotopesThrough calculations done, it was clear

that there are groups of atoms that are:of the same element but different masswhich 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 isotopesUsing 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; andLess 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.

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…

The Mass Spectrometer

What Aston Showed Us

Over 50 elements consisted of atoms of:the same atomic number but different relative atomic masshowever 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.

ReferencesReferences

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