Nuclear Chemistry 1.ppt

CMS

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 themass defect.
•Themass defect
•Δm=[Z(mp+me)+(A–Z)m
n]–m
atom
where m
p=mass of a proton; m
n=mass of
a neutron; m
e=mass of an electron;
m
atom=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
16
O on = 15.9949148 amu
•Therefore, mass defect = 0.141394 amu16
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 = mc
2
=0.141394 x 10
-3
kg x (3 x 10
8
ms
-1
)
2
/6.023 x 10
23
= 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

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.
•
56
Fe 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.

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.

16. variation of packing fraction with mass number.

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)