Transcript Atom and Nucleus. Radioactivity. Nuclear Energy.

Lecture 13
Atom and Nucleus. Radioactivity.
Nuclear Energy.
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Rutherford Model of the Atom
Radioactivity
Nuclear Energy
Binding Energy
Rutherford Model of the Atom
J.J. Thomson discovered electron in 1897.
He suggested an atomic model in which electrons were
embedded in spread positive charge.
Ernest Rutherford attempted to test this model by
bombarding a thin gold foil with alpha-particles.
A significant scattering of the alpha-particles was
detected.
Rutherford suggested a new model of the atom which
resembled a mini solar system.
Nuclear Structure
The nucleus contains protons and neutrons (discovered
in 1932). They both are referred to as nucleons.
Atoms of different chemical elements have different
number of protons (atomic number)
Atoms with the same number of protons and different
number of neutrons are called isotopes
A nucleus with a particular composition is called a nuclide.
1 H – hydrogen atom with 1 proton in the nucleus
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Radioactivity
Radioactivity was discovered accidentally in 1896.
A.Becquerel found that uranium (U) exposed
photographic film.
In other words, uranium emitted penetrating radiation.
Marie and Pierre Curie soon discovered 2 more radioactive
elements, which were called polonium and radium.
Radioactivity is associated with the nucleus and is not
affected by chemical reactions or heating.
Radioactive Decay
It was found that magnetic field splits the radiation
from radioactive materials into 3 parts.
Alpha ()particles  positively charged
Beta () particles  negatively charged  electrons
Gamma () particles (rays)  no charge
The nucleus composition does not change in gamma
decay, but changes in alpha- and beta-decay.
-decay occurs in large nuclei, which are not stable.
The heaviest stable isotope is 20983Bi.
-decay transforms a neutron into a proton (n  p+ + e)
Electron capture: p+ + e  n and p+  n + e+
Half-Life
The half-life of a radionuclide is the period of time needed
for half of any initial amount of the nuclide to decay.
1 mg 22688Ra  0.5 mg 22286Rn for 1600 years
Radionuclides have unchanging half-life.
This is the base for archaeological dating.
Half-life for 14C is 5730 years.
Radiation from radionuclides carries high energy.
It is harmful for living tissues.
1 sievert (Sv)  1 J of radiation from X-rays or -rays
absorbed by 1 kg of body tissue.
A permissible limit is ~ 20mSv a year.
Nuclear Energy
Our everyday life units for energy and mass are not
suitable for atoms.
The atomic mass unit: 1u = 1.66 1027 kg
Mass of a hydrogen atom is 1.0078 u
The energy unit is the electronvolt.
This is the energy an electron gains being accelerated
by a potential difference of 1 V.
1eV = 1.60 1019 J
1Mev = 1.60 1013 J
E (1 u) = mc2 = 931 MeV
Binding Energy
Hydrogen has 3 isotopes: protium, deuterium, and tritium.
The mass of 11H is 1.0078 u , the mass of a neutron is
1.0087 u . The sum is 2.0165 u .
However, the mass of a deuterium atom is 2.0141 u ,
0.0024 u = 2.2 MeV less than the combined mass of p+n.
All atoms have smaller masses than the sum of masses
of their nucleons.
Some mass and energy is given off due to action of
forces that hold the protons and neutrons together.
This energy is called binding energy.
Binding Energy
Binding energy of a deuterium atom is 2.2 MeV
(1.1 MeV per nucleon) ,
of a 20983Bi is 1640 MeV (7.8 MeV per nucleon).
A typical binding energy is ~800 billion kJ/kg
To boil water it takes 2260 kJ/kg
The most stable element is iron (5626Fe).
Fission and Fusion
If a heavy nucleus is split into 2 medium-size ones, each of
the new nuclei will have more binding energy per nucleon.
Joining 2 light nuclei would result in a nucleus with more
binding energy per nucleon.
The extra energy will be given off.
Splitting a large nucleus is called nuclear fission.
Joining light nuclei is called nuclear fusion.
Summary
Atomic nuclei consist of protons and neutrons.
Large nuclei are unstable and radioactively decay.
Radiation released by this process is dangerous
for living creatures.
The binding energy curve allows for two types of
nuclear reactions: fusion of light elements and
fission of heavy elements.