Friday, January 18, 2008

IIt JEE Ch.12. NUCLEAR CHEMISTRY - Core Points for Revision

JEE Syllabus

Nuclear chemistry:

isotopes and isobars;
Properties of a, b and g rays;
Kinetics of radioactive decay (decay series excluded),
carbon dating;
Stability of nuclei with respect to proton-neutron ratio;
Brief discussion on fission and fusion reactions.

The phenomenon of spontaneous emission ofactive radiations from certain substances is called radioactivity and the substances which emit such radiations are called radioactive substances.

Isotopes and Isobars

Atoms of the same element having same atomic number but different mass numbers are called isotopes.

Ex:(235, 92)U (238, 92)U

The atoms of different elements having different atomic numbers but same mass numbers are called isobars.

Ex (40,18)Ar ,(40,19)K , (40,20)Ca

An α particle contains two protons and two neutrons (and is similar to a He nucleus: ).

Beta Radiation (β) is the transmutation of a neutron into a proton and a electron (followed by the emission of the electron from the atom's nucleus:).

Gamma Radiation (γ) involves the emission of electromagnetic energy (similar to light energy) from an atom's nucleus.

Neutron-proton ratio

In the stability zone, for nuclei having atomic number up to 20, the neutron-proton ratio (n/p ratio) is close to unity.

for nuclei having atomic number more than 20, the n/p ratio for stability exceeds unity and goes up to 1.5 for heavier nuclei.

Radioactive decay proceeds according to a principal called the half-life. The half-life (T½) is the amount of time necessary for one-half of the radioactive material to decay.

During the fission of U235, three neutrons are released in addition to the two daughter atoms. If these released neutrons collide with nearby U235 nuclei, they can stimulate the fission of these atoms and start a self-sustaining nuclear chain reaction.As uranium atoms continue to split, a significant amount of energy is released from the reaction. The heat released during this reaction is harvested and used to generate electrical energy.

Nuclear fusion: reactions in which two or more elements "fuse" together to form one larger element, releasing energy in the process. A good example is the fusion of two "heavy" isotopes of hydrogen (deuterium: H2 and tritium: H3) into the element helium.

Carbon dating: The technique is based on the fact that all living matters contain a definite amount of radioactive isotope carbon 14 and after death decay of carbon 14 takes place. The amount of Carbon 14 remaining in the dead matter is determined from this the age of the body is calculated.

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