Transcript Document

Basic Nuclear Physics
Basic Atomic Structure
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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
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Atom
Nucleus
Electron Binding Energy
Periodic Table of the Elements
Isotopes
Nuclear Stability
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Atom
The atom is
composed of:
 positively charged
(+) protons,
 uncharged neutrons
and
 negatively charged
(-) electrons
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Atom
Thomson’s Model
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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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Summary of the Atom
Particle
Symbol
Proton
p
1.672E-27
938.2
+1
Neutron
n
1.675E-27
939.2
0
Electron
e
0.911E-30
0.511
-1
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Mass (kg) Energy (MeV) Charge
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 amu
Neutron = 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
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10 Most Abundant Elements
Element
Symbol
Protons
Relative % of Earth’s Mass
Oxygen
O
8
46.6
Silicon
Si
14
27.7
Aluminum
Al
13
8.1
Iron
Fe
26
5.0
Calcium
Ca
20
3.6
Sodium
Na
11
2.8
Potassium
K
19
2.6
Magnesium
Mg
12
2.1
Titanium
Ti
22
0.4
Hydrogen
H
1
0.1
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Periodic Table of the Elements
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).
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Periodic Table of the Elements
Lanthanide Series
Rare Earth
Elements
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Actinide Series
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Sample Element - Zirconium
Electron Shell Configuration:
40
Zr
Zirconium
91.2
K
L
M
N
O
1s-2
2s-2
3s-2
4s-2
5s-2
10 +
2p-6
3p-6
4p-6
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3d-10
4d- 2
+
2
8
18
10
2
12 = 40
K
L
M
N
O
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Sample Element - Zirconium
Name:
Symbol:
Atomic Number:
Atomic Mass:
Melting Point:
Boiling Point:
No. of Protons/Electrons:
No. of Neutrons:
Classification:
Phase at Room Temperature:
Density @ 293 K:
Color:
IAEA Date of Discovery:
Discoverer:
Zirconium
Zr
40
91.224 amu
1852.0 °C
4377.0 °C
40
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Transition Metal
Solid
6.49 g/cm3
Grayish
1789
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.
isotope notation
typically written as: A Xy
Z
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Xy = element symbol
A = atomic mass (neutron + protons)
Z = atomic number (protons)
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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
equal number of protons and neutrons
There are many
isotopes. Most have
more neutrons than
protons. Some are
stable but most are
unstable
(radioactive).
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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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The Line of Stability
N>Z
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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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Nuclear Stability
The nuclear force is an extremely shortrange 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
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Unstable Nuclei
 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
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Unstable Nuclei
 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
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Some Common Radionuclides
Naturally occurring 235U and 238U
60Co, 137Cs, 90Sr found in nuclear power
plants
192Ir used in radiography
99mTc used in nuclear medicine
131I 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)
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