Nuclei- 1 Flashcards
1 amu
1 atomic mass unit or unifieid mass is 1/2th mass of 6C12 isotope of carbon
isotopes
isobars
isotones
isotopes- same atomic number- h1 h2 h3
The element gold has 32 isotopes, ranging from A =173 to
A = 204.
isobars- same mass number- k-40, ar -40
isotones- same no of neutrons cl37, k-39
discovery of neutron
- This hypothesis was verified in 1932 by James Chadwick
who observed emission of neutral radiation when beryllium nuclei were
bombarded with alpha-particles
-A free neutron, unlike a free proton, is unstable. It decays into a
proton, an electron and a antineutrino (another elementary particle), and
has a mean life of about 1000s. It is, however, stable inside the nucleus.
density
Thus the
density of nucleus is a constant, independent of A, for all nuclei. Different
nuclei are like a drop of liquid of constant density. The density of nuclear
matter is approximately 2.3 × 1017 kg m–3
mass energy
it was presumed that mass and energy were conserved
separately in a reaction. However, Einstein showed that mass is another
form of energy and one can convert mass-energy into other forms of
energy, say kinetic energy and vice-versa.
what is binding energy
Binding energy of the nucleus is the energy with which the nucleons are bound in the nucleus.
If all the protons and neutrons of a nucleus are brought together, an energy of Eb will be released.
what is mass defect
The difference between the sum of the masses of neutrons and protons forming the nucleus and the actual mass of the nucleus is called mass defect.
Ebn
binding
energy per nucleon, Ebn, which is the ratio of the binding energy Eb
of a
nucleus to the number of the nucleons, A, in that nucleus:
Main features of Ebn v/s A plot
(i) the binding energy per
nucleon, Ebn, is practically
constant, i.e. practically
independent of the atomic
number for nuclei of middle
mass number ( 30 < A < 170).
The curve has a maximum of
about 8.75 MeV for A = 56
and has a value of 7.6 MeV
for A = 238.
(ii) Ebn is lower for both light
nuclei (A<30) and heavy
nuclei (A>170).
why is nuclear force constant from 30<a<170
- The constancy of the binding energy in the range 30 < A < 170 is a consequence of the fact that the nuclear force is short-ranged.
- Consider a particular nucleon inside a sufficiently large nucleus. It will be under the influence of only some of its neighbours, which come within the range of the nuclear force. If any other nucleon is at a distance more than the range of the nuclear force from the particular nucleon it will have no influence on the binding energy of the nucleon under
consideration.
-If a nucleon can have a maximum of p neighbours within the range of nuclear force, its binding energy would be proportional to p. Let the binding energy of the nucleus be pk, where
k is a constant having the dimensions of energy. If we increase A by adding nucleons they will not change the binding energy of a nucleon
inside
-The binding energy per nucleon is a constant and is approximately equal to pk. The property that a given nucleon influences only nucleons close to it is also referred to as saturation
property of the nuclear force.
explain why energy is released in nuclear fission
(i) When we move from the heavy nuclei region to the middle region of the plot, we find that there will be a gain in the overall binding energy and hence release of energy. This indicates that energy can be released
when a heavy nucleus (A = 240) breaks into two roughly equal fragments. This process is called Nuclear
Fission.
Why is energy released in nuclear fusion?
(ii) Similarly, when we move from lighter nuclei to heavier nuclei, we again find that there will be a gain in the overall binding energy and hence release of energy. This indicates that energy can be released when two or more lighter nuclei fuse together to form a heavy nucleus. This process is called Nuclear Fusion.
