Download Atomic Nucleus web

28/10/2015
The structure of the
atomic nucleus
06.10.2015.
Discovery of the atomic nucleus
Rutherford’s gold foil experiment (1911)
1
28/10/2015
Discovery of the proton
• 1917 – Ernest Rutherford proved that hydrogen nucleus is
present in other nuclei
•
+
(α−particle) →
+
(nucleus
of
hydrogen)
• Πρῶτον: "first“ (lightest element)
Discovery of the
neutron
• Sir James Chadwick (1932)
• 1935 - Nobel Prize in
Physics for his discovery of
the neutron in 1932
+
→
+
2
28/10/2015
Nuclear Binding Energy
• nucleon: a constituent (proton or neutron) of an atomic
nucleus.
• When nucleus is formed, the mass of nucleons forming it
decreases = mass defect -> provides the Binding Energy
• Binding energy: the amount of work required to separate
the nucleons at infinite distance.
• The binding energy/nucleon is a measure of stability of
nucleus.
• Properties of the nuclear force:
•
•
•
•
high intensity (strong)
short-range (10-15m)
always (!) attractive force
independent of electric charge (neutrons are also affected)
Nuclear Binding Energy
• 2p + 2n = nucleus of Helium (ion)
• 2p = 2*1.6726x10-27kg = 3.3452x10-27kg
6.695x10-27kg
• 2n = 2*1.6749x10-27kg = 3.3498x10-27kg
• mass of the nucleus of Helium = 6.6447x10-27kg
• mass defect = 5.03x10-29kg => 4.5207x10-12J (E=mc2)
• ionization energy of the hydrogen atom: 2.18 × 10−18J
• Enuclear/Eelectron ~ 2 million
3
28/10/2015
Binding energy curve
Models describe the structure of
the atomic nucleus
• Liquid Drop Model (Carl-Friedrich von Weizsäcker-1935)
• Nuclear Shell Model (Maria Goeppert-Mayerin-1948)
4
28/10/2015
Liquid Drop Model
• The nucleus is a drop (spherical) of incompressible
nuclear fluid
• The fluid is made of nucleons (protons and neutrons),
which are held together by the strong nuclear force
• nuclei have approximately constant density
• components of the binding energy (EB) of the nucleus
• volume energy (EB is directly proportional to the A)
• surface energy (nucleons on the surface have smaller EB)
• Coulomb energy (electric repulsion between protons
decrease the binding energy)
• asymmetry energy (Pauli energy) (the same number of
neutrons as protons produce more stable form of nuclear
matter)
• pairing energy (even number of particles is more stable)
The Carl-Friedrich von Weizsäcker
semiempirical mass formula (1935)
=
−
−
!
−
− 2!
"
±δ
,!
5
28/10/2015
Nuclear Shell Model
• Maria Goeppert-Mayerin - 1948
• arrangements of the fundamental particles (proton, neutron) in the
different energy shells (levels)
• describe the structure of the nucleus in terms of energy levels
• the shells for protons and for neutrons are independent of each
other
• nuclei with certain number of nucleons have higher binding
energies than the previous or the following nuclues (supports the
shell model!!!)
• → closed nuclear shells
• magic numbers of nucleons: 2, 8, 20, 28, 50, 82, 126
• The stable elements at the end of the naturally occuring radioactive
series all have a "magic number" of neutrons or protons.
Nuclear stability
•
•
•
•
Up to atomic number 20, n=p is stable
Above atomic number 20, n>p stable
Above atomic number 84, all nuclei are unstable
Nuclei with magic numbers of protons and/or
neutrons are stable
• even number of protons and neutrons are more
stable.
6
28/10/2015
• The end
7