B.Sc. SY, Sem-III, Paper No. III�Unit.I : Nuclear Chemistry

Definition : Nuclear chemistry is a branch of chemistry which deals with the study of nuclear particles, nuclear forces and nuclear reactions.

• Classification Of Nuclides : Nuclides are classified into following types

i) Isotopes :

Definition : Isotopes are the atoms of the same elements which have the same atomic number (Z) but different mass number (A).

• Examples: i) ¹H₁ , ²H₁ , ³H₁, ²D₁, or ³T₁ are the isotopes of hydrogen

^{ii) 12}C₆ , ¹⁴C₆ iii) ¹⁶O₈, ¹⁷O_8, ¹⁸O₈

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• ii) Isobars: Isobars are the atoms of different elements having same mass number but different atomic numbers.

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• Examples: i) ⁴⁰Ar_18, ii) ⁴⁰K_{19 ,} iii) ⁴⁰Ca_20.
• Iii) Isotones : Isotones are the atomic species possess different atomic number or protons and also different mass number.

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• Examples: i) ³⁰ Si₁₄ ii) ³¹P₁₅ iii) ³²S₁₆ are the isotones

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• Iv) Isomers : Atoms having the same mass number (A) and same atomic number (Z) but different radioactive properties are called isomers.

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• Examples : i) ²³⁴U₉₂ ii) ²³⁴U₉₂

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• Atomic nucleus and its composition:

i) The existence of nucleus in the atom was first of all proposed by E. Rutherford in the year 1911.

ii) Rutherford’s basic idea that an atom contains dense, positively charged nucleus now is accepted universely.

iii) The effective diameter of an atom is about 10^-8 cm.

iv) In contrast the nucleus has a diameter of only about 10^-13 cm.

v) Generally the nuclei are nearly spherical in shape but most of the exceptions occur in rare earth elements in which shape is slightly ellipsoidal.

vi) The protons and neutrons which constitute the nucleus are collectively known as nucleons.

vi) The total number of nucleons is denoted by ‘A’ and is called mass number of the atom.

Mass Number = Number of protons + Number of neutrons.

A = p + n

vii) The nucleus of any atom may be represented by specifying the atomic number as a subscript at the left hand bottom of the atomic symbol and mass number as a subscript at the left hand top of the symbol.

For. Example. ¹²c₆

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• Nuclear Stability :

1) Odd and Even number of protons and neutrons: -

i) At present more than 900 nuclides are known, out of these only 272 nuclides are stable, while others are radioactive.

ii) On the basis of odd and even number of protons and neutrons these 272 nuclides have been grouped into four classes which are shown in the following table.

iii) This table shows that the number of even-p-odd-n and odd –p- and even- n type nuclei are nearly same the in number.

iv) While, the nuclei of odd-p-odd-n type are only four and hence not found in nature. These nuclei are limited only to light nuclei.

v) The maximum number of stable nuclei are of even-p-even-n type.

vi) 85% of the stable isotopes are found in earth’s crust are of even-p-even-n type, while the remaining 15% isotopes are of odd-p-even-n type.

vii) The presence of even –p- even –n type isotopes in the maximum % in the earth’s crust shows that the stable nuclei have a tendency to form p-p and n-n pairs.S

2) Magic Number :

This model was proposed by Maria Mayer, Enene Wigner and Hans Jensen in 1950 which earned Noble prize in 1963.

According to this model-

i) Each neutron moves in its orbit within the nucleus independently of other nucleons. For this reason this model is also known as Independent particle model or Single particle model.

ii) Each nucleon is acted up on by an average field produced by the action of other nucleons.

iii) The neutrons as well as protons pair amongst themselves (i.e. a neutron forms a pair with another proton and a proton forms a pair with another proton) in the nucleus.

iv) These paired electrons are packed into separate shells called nuclear shells within the nucleus.

v) A nuclear shell which contains a specific number of protons or neutrons in it is called a filled shell, this filled shell is more stable than other shells.

vi) These specific numbers are- 2,8,20,28,50,82 and 126 which are called magic numbers.

vii) These magic numbers indicates that the nucleons exist in pairs in nucleus.

viii) The nuclei having either protons or neutrons or both equal to the magic numbers given above are known as- magic nuclei or noble nuclei these nuclei are also very stable.

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3) Packing fraction (f) :- The difference in atomic mass of an isotope and mass number was expressed by F.W. Aston in 1927 as packing fraction by the following expression.

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Packing fraction (f) = isotopic mass – mass number

^{Mass number} 10⁴

⁼ M - A

10⁴

A