lecture 1 basic nuclear physics 1 - basic atomic structure
TRANSCRIPT
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Basic Nuclear Physics
Basic Atomic Structure
Day 1- Lecture 1
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Objective
• To discuss about the structure of the atom including the Neutron, Proton and Electron
• To learn about the Atomic Number, Atomic Mass, nuclear stability and radioactive or unstable nuclei
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Contents
• Atom• Nucleus• Electron Binding Energy• Periodic Table of the Elements• Isotopes• Nuclear Stability
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Atom
positively charged (+) protons,
uncharged neutrons and
negatively charged (-) electrons
The atom iscomposed of:
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Atom
Thomson’s Model Rutherford’s Model
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Atom
Bohr’s Model
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Nucleus
Protons and neutrons together form the nucleus of the atom.
The nucleus determines the identity of the element and its atomic mass.
Proton and neutrons have essentially the same mass but only the proton is charged while the neutron has no charge.
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Protons
Protons are positively charged particles found inside the nucleus of an atom. Each element has a unique atomic number (a unique number of protons).
Proton number never changes for any given element. For example, oxygen has an atomic number of 8 indicating that oxygen always has 8 protons.
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Neutrons
Neutrons are the other particle found in the nucleus of an atom. Unlike protons and electrons, however, neutrons carry no electrical charge and are thus "neutral."
Atoms of a given element do not always contain the same number of neutrons.
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Electrons
Electrons are negatively charged particles that surround the nucleus in “orbits” similar to moons orbiting a planet.
The sharing or exchange of electrons between atoms forms chemical bonds which is how new molecules and compounds are formed.
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ELECTRON BINDING ENERGY
• Electrons exist in discrete “shells” around the nucleus (similar to planets around the sun)
• Each shell represents a unique binding energy holding the electron to the nucleus
• The shells are designated by letters (K, L, M, N …) where K, the shell closest to the nucleus, has the largest binding energy, so the K electron is the most tightly bound
• Maximum number of electrons in each shell: 2 in K shell, 8 in L shell …
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Particle Symbol Mass (kg) Energy (MeV) Charge
Proton p 1.672E-27 938.2 +1Neutron n 1.675E-27 939.2 0Electron e 0.911E-30 0.511 -1
Summary of the Atom
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Atomic Mass Unit (amu)
Where 1 amu is approximately equal to
1.6605 x 10-24 grams
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Atomic Mass Unit (amu)
The atomic mass of the proton and the neutron is approximately:
Proton = 1.6726 x 10-24 grams = 1.0073 amuNeutron = 1.6749 x 10-24 grams = 1.0087 amu
Thus, the neutron is just a little heavier than the proton.
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Atomic Mass Unit (amu)
The difference in the mass of the neutron and the proton can be understood if we assume that the neutron is merely a proton combined with an electron forming a neutral particle slightly more massive than a proton alone.
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Atomic Mass Unit (amu)
The atomic mass of the electron is approximately:
Electron = 9.1094 x 10-28 grams = 0.00055 amu
Thus, the electron has a much smaller mass than either the proton or the neutron, 1837 times smaller or about 2000 times smaller.
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Elements
The number of protons in an atom dictate the element.
For an uncharged atom, the number of electrons equals the number of protons.
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10 Most Abundant Elements
Element Symbol Protons Relative % of Earth’s MassOxygen O 8 46.6Silicon Si 14 27.7
Aluminum Al 13 8.1Iron Fe 26 5.0
Calcium Ca 20 3.6Sodium Na 11 2.8
Potassium K 19 2.6Magnesium Mg 12 2.1
Titanium Ti 22 0.4Hydrogen H 1 0.1
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In 1869, Russian chemist Dmitri Mendeleev first described an arrangement of the chemical elements now known as the periodic table.
The periodic table displays all chemical elements systematically in order of increasing atomic number (the number of protons in the nucleus).
Periodic Table of the Elements
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Rare EarthElements
Actinide Series
Lanthanide Series
Periodic Table of the Elements
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Sample Element - Zirconium
40Zr
Zirconium91.2
Electron Shell Configuration:
K 1s-2 2L 2s-2 2p-6 8M 3s-2 3p-6 3d-10 18N 4s-2 4p-6 4d- 2 10O 5s-2 2
10 + 18 + 12 = 40KL
MNO
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Sample Element - Zirconium
Name: Zirconium Symbol: Zr
Atomic Number: 40 Atomic Mass: 91.224 amu Melting Point: 1852.0 °C Boiling Point: 4377.0 °C
No. of Protons/Electrons: 40 No. of Neutrons: 51
Classification: Transition MetalPhase at Room Temperature: Solid
Density @ 293 K: 6.49 g/cm3 Color: Grayish
Date of Discovery: 1789 Discoverer: Martin Klaproth
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Isotopes
Atoms of an element that have a different number of neutrons in the nucleus are called isotopes of each other.
XyZA Xy = element symbol
A = atomic mass (neutron + protons) Z = atomic number (protons)
isotope notation typically written as:
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Isotopes
The number of protons and electrons remain the same.
But the number of neutrons varies.
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Isotopes
There are many isotopes. Most have more neutrons than protons. Some are stable but most are unstable (radioactive).
equal number of protons and neutrons
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Nuclear Stability• A stable or non-radioactive nuclide is one
whose atoms do not decay
• If one plots the stable nuclei, an interesting pattern emerges (shown in next slide)
• The graph in the next slide shows a plot of neutron number N vs atomic number Z for the stable nuclei
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N > Z
The Line of Stability
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Nuclear Stability
For the heaviest stable nuclei, N is about 1.5 times Z
The presence of the extra neutrons overcomes the positively charged protons’ tendency to repel each other and disrupt the nucleus
The nucleus is held together by a poorly understood force, the Nuclear Force
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The nuclear force is an extremely short-range force
It acts over a maximum distance of about two proton diameters
The nuclear force is responsible for the binding energy that holds the nucleus together
Nuclear Stability
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Nuclei which do not fall on the line of stability tend to be unstable or “radioactive”
They are called “radionuclides”
A few radionuclides do fall on the line of stability but their rate of decay is so slow that for all practical purposes they are stable
Unstable Nuclei
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Radionuclides undergo a process called radioactive transformation or disintegration
In this process, the nucleus emits particles to adjust its neutron (N) to proton (Z) ratio
This change in the N to Z ratio tends to move the radionuclide toward the line of stability
Unstable Nuclei
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Some Common Radionuclides
Naturally occurring 235U and 238U60Co, 137Cs, 90Sr found in nuclear power
plants192Ir used in radiography 99mTc used in nuclear medicine131I used in treatment of thyroid conditions
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Summary
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Where to Get More Information
Cember, H., Johnson, T. E, Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2009)
International Atomic Energy Agency, Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources (PGEC), Training Course Series 18, IAEA, Vienna (2002)