IAEA 1

Basic Nuclear Physics

Basic Atomic Structure

Day 1- Lecture 1

IAEA 2

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

IAEA 3

Contents

• Atom• Nucleus• Electron Binding Energy• Periodic Table of the Elements• Isotopes• Nuclear Stability

IAEA 4

Atom

positively charged (+) protons,

uncharged neutrons and

negatively charged (-) electrons

The atom iscomposed of:

IAEA 5

Atom

Thomson’s Model Rutherford’s Model

IAEA 6

Atom

Bohr’s Model

IAEA 7

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.

IAEA 8

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.

IAEA 9

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.

IAEA 10

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.

IAEA

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 …

IAEA

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

IAEA 13

Atomic Mass Unit (amu)

Where 1 amu is approximately equal to

1.6605 x 10-24 grams

IAEA

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.

IAEA 15

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.

IAEA

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.

IAEA

Elements

The number of protons in an atom dictate the element.

For an uncharged atom, the number of electrons equals the number of protons.

IAEA 18

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

IAEA 19

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

IAEA 20

Rare EarthElements

Actinide Series

Lanthanide Series

Periodic Table of the Elements

IAEA 21

IAEA 22

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

IAEA 23

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

IAEA 24

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:

IAEA 25

Isotopes

The number of protons and electrons remain the same.

But the number of neutrons varies.

IAEA 26

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

IAEA 27

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

IAEA 28

N > Z

The Line of Stability

IAEA 29

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

IAEA 30

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

IAEA 31

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

IAEA 32

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

IAEA 33

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

IAEA 34

Summary

IAEA 35

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)