PROVING THE PRESENCE OF ISOTOPES By Rania, Cheryl, ZhenJie and Ivy

Definition of Isotopes 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. 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).

Examples of Isotopes Similarities? Differences? Carbon Protons Neutrons Nucleus Carbon Carbon atom A: 126C – This carbon atom has a mass number of 12, and has an atomic number of 6. Therefore, it has 6 protons, and 6 neutrons. Carbon atom B: 136C – This carbon atom has a mass number of 13, and a mass number of 6. Therefore it has 6 protons, and 7 neutrons. Conclusion: These 2 carbon atoms A & B, have the same number of protons, but a different number of neutrons. Therefore, this pair of carbon atoms are isotopes of each other. 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. Mass spectrometry is also used to determine the isotopic composition of elements within a sample. Differences in mass among isotopes of an element are very small, and the less abundant isotopes of an element are typically very rare, so a very sensitive instrument is required.

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). This is an example of how a mass spectrometer works. Let’s use sodium chloride (NaCl) as the example. In ion source, the substance is turned into gas and changed into electrically charged particles, to form Sodium (Na+) & Chlorine (Cl-). Chlorine atoms and ions come in two isotopes. One’s mass is approximately 35 amu, with an abundance of 75%, and the other is approximately 37 amu, with an abundance of 25%.

The Mass Spectrometer After the substance is charged, it is being repelled into the spectrometer. This is an example of how a mass spectrometer works. Let’s use sodium chloride (NaCl) as the example. In ion source, the substance is turned into gas and changed into electrically charged particles, to form Sodium (Na+) & Chlorine (Cl-). Chlorine atoms and ions come in two isotopes. One’s mass is approximately 35 amu, with an abundance of 75%, and the other is approximately 37 amu, with an abundance of 25%.

The Mass Spectrometer The analyzer (which exerts electromagnetic force) bends the ray of ions, as there is a (positive) charge. The part of the spectrometer called the analyzer has an electric and magnetic field, that exerts forces on ions which are travelling through these fields.

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. By Newton’s Second Law of Motion, lighter ions get deflected by the magnetic force more than heavier ions. The speed of a charged particle travelling through the fields may change, & their direction may be changed by the magnetic forces.

The Mass Spectrometer As some isotopes are heavier, their speed will become slower than those which are lighter due to the repelling force. The speed of a charged particle travelling through the fields may change, & their direction may be changed by the magnetic forces.

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 extent of the deflection of the moving ion’s path depends on it’s mass-to-charge ratio.

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… This is an example of how a mass spectrometer works. Let’s use sodium chloride (NaCl) as the example. In ion source, the substance is turned into gas and changed into electrically charged particles, to form Sodium (Na+) & Chlorine (Cl-). Chlorine atoms and ions come in two isotopes. One’s mass is approximately 35 amu, with an abundance of 75%, and the other is approximately 37 amu, with an abundance of 25%. Sodium atoms and ions have a mass of approximately 23 amu.

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=wKzJTBZh20wC&pg=PA234&dq=evidence+of+the+presence+of+isotopes&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 THANK YOU!