Sunday, October 21, 2007


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.

Syllabus rearranged

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




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

In 1896, Henri Becquerel expanded the field of chemistry to include nuclear changes when he discovered that uranium emitted radiation.

Soon after Becquerel's discovery, Marie Curie began studying radioactivity and completed much of the pioneering work on nuclear changes. Curie found that radiation was proportional to the amount of radioactive element present, and she proposed that radiation was a property of atoms (as opposed to a chemical property of a compound).

Marie Curie was the first woman to win a Nobel Prize and the first person to win two (the first, shared with her husband Pierre and Becquerel for discovering radioactivity; the second for discovering the radioactive elements radium and polonium).

Isotopes and Isobars

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

Ex: 92235U 92238U

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

Ex 1840Ar , 1940K , 2040Ca

Radiation and Nuclear Reactions

In 1902, Frederick Soddy proposed the theory that "radioactivity is the result of a natural change of an isotope of one element into an isotope of a different element." Nuclear reactions involve changes in particles in an atom's nucleus and thus cause a change in the atom itself. All elements heavier than bismuth (Bi) (and some lighter) exhibit natural radioactivity and thus can "decay" into lighter elements. Unlike normal chemical reactions that form molecules, nuclear reactions result in the transmutation of one element into a different isotope or a different element altogether (remember that the number of protons in an atom defines the element, so a change in protons results in a change in the atom).

There are three common types of radiation.Alpha Radiation (α),Beta Radiation (β),Gamma Radiation (γ).


Alpha Radiation (α) is the emission of an alpha particle from an atom's nucleus.

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

When an atom emits an a particle, the atom's atomic mass will decrease by four units (because two protons and two neutrons are lost) and the atomic number (z) will decrease by two units.

The element is said to "transmute" into another element that is two z units smaller.

An example of an a transmutation takes place when uranium decays into the element thorium (Th) by emitting an alpha particle.

The nuclei of atoms represented by their atomic numbers and mass numbers are called nucleides.

For example 92235U

Note: in nuclear chemistry, element symbols are traditionally preceded by their atomic weight (upper left) and atomic number (lower left).

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:).

When an atom emits a β particle, the atom's mass will not change (since there is no change in the total number of nuclear particles), however the atomic number will increase by one (because the neutron transmutated into an additional proton).

An example of this is the decay of the isotope of carbon named carbon-14 into the element nitrogen:

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

No particles are emitted during gamma radiation, and thus gamma radiation does not itself cause the transmutation of atoms, however γ radiation is often emitted during, and simultaneous to, α or β radioactive decay.

X-rays, emitted during the beta decay of cobalt-60, are a common example of gamma radiation.

Stability of nuclei with respect to proton-neutron ratio

It has been observed that the stability of nucleus depends upon the neutron to proton rati (n/P). In a plot of neutrons versus protons for nuclei of various elements, it has been observed that most th stable (non0radio active) elements lie in a belt, termed as stability belt or zone.

In the stability zone, for nuclei having atomic number up to 20, the 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.

Kinetics of Radioactive Decay

Half-Life Concept

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.

For example, the radioactive element bismuth (210Bi) can undergo alpha decay to form the element thallium (206Tl) with a reaction half-life equal to five days.

If we begin an experiment starting with 100 g of bismuth in a sealed lead container, after five days we will have 50 g of bismuth and 50 g of thallium in the jar. After another five days (ten from the starting point), one-half of the remaining bismuth will decay and we will be left with 25 g of bismuth and 75 g of thallium in the jar. As illustrated, the reaction proceeds in halfs, with half of whatever is left of the radioactive element decaying every half-life period.

Stability of nuclei with respect to proton-neutron ratio

The stability of nucleus depends upon the neutron to proton ratio.

When the number of neutrons (n) are plotted against number of protons (p) for nuclei of various items, it has been observed that most of the stable (non-radio active) nuclei fall in a zone or belt. This zone is called stability zone or belt.

a. Fornuclei having atomic number up to 20 the stability belt(n/p ratio) is very close to one.
b. For nuclei having atomim number more than 20, the ratio increases up to 1.5 for heavier nuclei.
c. When the ratio is different from the stability ratio, the nuclei emit alpha or beta particles to move into stability zone.

Nuclear fission: reactions in which an atom's nucleus splits into smaller parts, releasing a large amount of energy in the process. Most commonly this is done by "firing" a neutron at the nucleus of an atom. The energy of the neutron "bullet" causes the target element to split into two (or more) elements that are lighter than the parent atom.

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. This chain reaction is the basis of nuclear power. 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.

Fusion reactions release tremendous amounts of energy and are commonly referred to as thermonuclear reactions. Although many people think of the sun as a large fireball, the sun (and all stars) are actually enormous fusion reactors. Stars are primarily gigantic balls of hydrogen gas under tremendous pressure due to gravitational forces. Hydrogen molecules are fused into helium and heavier elements inside of stars, releasing energy that we receive as light and heat.

Carbon dating

Carbon dating is one of the uses of radio active isotopes. The technique will help to find out the age of archaeological objects (wood, plant, and animal fossils). the principle is based on the fact that all living matters contain a definite amount of radioactive isotope carbon 14.

Carbon 14is formed in the upper atmosphere by the bombardment of N-14 by cosmic rays. Some of it is present in the carbon dioxide. In the photosynthesis process, plant absorb some C-14. As the animals live on plants they acquire some C-14. A plant or any living being bduirng its life time maintains a reasonable balance of C 14 in its tissues. From the death of the plant, it will not absorb any more c-14. But C-14 starts decaying.

Therefore by having an estimate of C-14 that will be normally there in a tissue and by determining the amount of C-14 now in the dead tissue, the age of the dead tissue can be determined.


web sites
The Physics of Radioactivity, Radioisotope uses

JEE Question 2007 paper II

A positron is emitted from [23,11]Na. The ratio of the atomic mass and atomic number of the resulting nuclide is

(A) 22/10
(B) 22/11
(C) 23/10
(D) 23/12

Solution: C

[23,11]Na --> [23,10]Ne + [0,+1]e
Hence ratio of atomic mass to atomic number of the resulting nuclide is 23/10

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