1. NUCLEAR CHEMISTRY
• In ordinary reactions electrons are
involved.
• Nucleus remains unaffected
• In nuclear reaction nucleus under go
changes.
• It is not common because nucleus is
highly stable.
2. • Atom = nucleus + electrons
• Nucleus = neutrons and protons held together by
“strong interactions”
• Strong nuclear force (interaction) is a
fundamental force of nature
• Range of force is about 10-15 m
• Strong enough to overcome Coulombic repulsion
of protons
3. • Stability of a nucleus can be explained in
terms of
• Mass defect
• Binding energy
• n/p ratio and
• Packing fraction
4. Mass defect
• The difference between the mass of an
atom and the sum of the masses of the
nucleons and electrons of which it is
composed is called the mass defect.
• The mass defect
• Δm = [Z(mp + me) + (A – Z)mn] – matom
where mp = mass of a proton; mn = mass of
a neutron; me = mass of an electron;
matom = Actual mass atom, Z = atomic
number, A = mass number
5. For example consider
•
– Mass of proton = 1.007825 amu
– Mass of neutron = 1.008665 amu
– Mass of electron = 0.0005485 amu
• Thus:
– 8 protons = 8.0626
– 8 neutrons = 8.06932
– 8 electrons = 0.004388
– Sum = 16.136308
• Actual mass of 16O on = 15.9949148 amu
• Therefore, mass defect = 0.141394 amu
16
8 O
6. Binding Energy of Nucleus
• Decrease in mass ie Mass defect is
converted to energy release when atom is
formed, according to Einstein’s equation
i.e.:
• E = mc2
= 0.141394 x 10-3 kg x (3 x 108 ms-1)2/6.023 x 1023
= 2.1128 x 10-11 J
• But 1 eV = 1.6021 x 10-19 J
• Thus E = 131.9 MeV
or Binding energy= 8.24 MeV per nucleon
7. Binding Energy of Nucleus
• Indication of how strongly the nucleus is bound
together
• Energy liberated in formation of nucleus from its
nucleons is a measure of its stability
• High binding energy = stable nucleus
• Plot binding energy per nucleon vs. mass number
is given in next slide
8.
9. • Greater the mass defect, greater is the
binding energy and greater is the stability.
• В.Е per nucleon first increases, reaches a
maximum and then decreases
• Binding energy of a stable nucleus varies
from 7-8 MeV.
• 56Fe have a binding energy per nucleon
value of approximately 8.8 MeV. It's one of
the most stable nuclides that exist.
10. • Isotopes having intermediate mass
numbers (between 40 & 60) are more
stable.
• Isotopes of low mass number and high
mass number are unstable
11. n/p Ratio
• n/p ratio represents the ratio of no: of neutrons to no: of
protons in an atom.
• When a graph is drawn between the number of neutrons
and no: of protons in the nucleus of different atoms, we
get a belt of stability or zone of stability.
• Elements with atomic number upto 20 have n/p ratio =1
• All elements which lie within the belt are stable.
12.
13. Packing Fraction
• The atomic masses (isotopic mass) of elements
are close to but not exactly equal to whole
numbers.
• But mass numbers are whole numbers.
• The variation of isotopic mass from whole
numbers is expressed in terms of packing
fraction.
• This variation occurs due to mass defect.
14. • Packing fractions can have negative or positive values.
• Negative value means that isotopic mass is less than
mass number and that some mass is lost during its
formation as energy.
• Hence greater the negative value of packing fraction,
greater is the binding energy and stability.
• Low value of packing fraction indicates greater stability.
15. • Positive value of packing fraction indicates
lesser stability.
• But this is always not true with elements of low
mass numbers.
• For example,
hydrogen, helium and carbon have positive
packing fractions, but they have low positive
values and are stable.
17. From the graph
• Packing fraction decreases with mass number.
But increases for heavy elements
• Elements with mass number near 45 have
lowest packing fractions. They are highly stable.
• Beyond mass number 200, packing fractions are
positive and these elements are unstable.
(radioactive)
