IAEA 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 2
IAEA Contents Atom Nucleus Electron Binding Energy Periodic Table of the Elements Isotopes Nuclear Stability 3
IAEA Atom positively charged (+) protons, uncharged neutrons and negatively charged (-) electrons The atom is composed of: 4
IAEA 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. 7
IAEA 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. 8
IAEA 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. 9
IAEA 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. 10
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 ParticleSymbolMass (kg)Energy (MeV)Charge Protonp1.672E-27938.2+1 Neutronn1.675E-27939.20 Electrone0.911E-300.511 Summary of the Atom
IAEA Atomic Mass Unit (amu) Where 1 amu is approximately equal to 1.6605 x 10 -24 grams 13
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 amu Neutron = 1.6749 x 10 -24 grams = 1.0087 amu Thus, the neutron is just a little heavier than the proton.
IAEA 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. 15
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 10 Most Abundant Elements ElementSymbolProtons Relative % of Earth’s Mass OxygenO846.6 SiliconSi1427.7 AluminumAl138.1 IronFe265.0 CalciumCa203.6 SodiumNa112.8 PotassiumK192.6 MagnesiumMg122.1 TitaniumTi220.4 HydrogenH10.1 18
IAEA 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 19
IAEA Rare Earth Elements Actinide Series Lanthanide Series Periodic Table of the Elements 20
IAEA Sample Element - Zirconium 40 Zr Zirconium 91.2 Electron Shell Configuration: K1s-22 L2s-22p-68 M3s-23p-63d-1018 N4s-24p-64d- 210 O5s-22 10 + 18 + 12 = 40 K L M N O 22
IAEA 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 Metal Phase at Room Temperature:Solid Density @ 293 K: 6.49 g/cm 3 Color: Grayish Date of Discovery: 1789 Discoverer: Martin Klaproth 23
IAEA Isotopes. Atoms of an element that have a different number of neutrons in the nucleus are called isotopes of each other. Xy Z A Xy = element symbol A = atomic mass (neutron + protons) Z = atomic number (protons) isotope notation typically written as: 24
IAEA Isotopes The number of protons and electrons remain the same. But the number of neutrons varies. 25
IAEA 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 26
IAEA 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 27
IAEA 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 29
IAEA 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 30
IAEA 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 31
IAEA 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 32
IAEA Some Common Radionuclides Naturally occurring 235 U and 238 U 60 Co, 137 Cs, 90 Sr found in nuclear power plants 192 Ir used in radiography 99m Tc used in nuclear medicine 131 I used in treatment of thyroid conditions 33
IAEA 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) 35
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