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Session I.2.1

Part I Review of Fundamentals

Module 2 Basic Physics and MathematicsUsed in Radiation Protection

Session 1 Basic Atomic Structure

IAEA Post Graduate Educational CourseRadiation Protection and Safety of Radiation Sources

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Overview

In this session we will discuss the building blocks of the atom including the Neutron, Proton and Electron

We will also discuss how the Atomic Number specifies the elements and how they are arranged in the Periodic Table

Finally, we will discuss how Isotopes of an element have different Atomic Mass Unit

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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 (i.e. a unique number of protons).

The number of protons 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 particles 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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Particle Symbol Mass (kg) Energy (MeV) Charge

Proton p 1.672E-27 938.2 +1

Neutron n 1.675E-27 939.2 0

Electron 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 mass of the electron is much smaller than that of either the proton or the neutron, about 2000 times smaller (precise value 1837)

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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 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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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 = 40

K

LMNO

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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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Stable Nuclides

long rangeelectrostatic

forces

short rangenuclear forces

p

p

n

Line of stability

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Stable and Unstable Nuclides

Too manyneutrons

for stability

Too manyprotons

for stability

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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 (2008)

Martin, A., Harbison, S. A., Beach, K., Cole, P., An Introduction to Radiation Protection, 6th Edition, Hodder Arnold, London (2012)

Jelley, N. A., Fundamentals of Nuclear Physics, Cambridge University Press, Cambridge (1990)

Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8th Edition, 1999 update), Wiley, New York (1999)