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Announcements & Agenda (01/22/07) (01/22/07) You should currently be reading Ch 4! Quiz on Wednesday over Ch 3! Today: Radiation (Ch 9 – only responsible for notes) Electron energy levels (3.7) Periodic trends (3.3, 3.8) Last Time: All Atoms of the Same Element Have the Same # of Protons!!! 11 protons Symbol 11 Na Also, the # of protons = the # of electrons for a neutral atom 1 Last Time: Nuclear Symbols & Isotopes 2 Last Time: The Atomic Mass is NOT the Same as the Mass Number • represents a particular isotope of an element. • gives the mass number in the upper left corner and the atomic number in the lower left corner. Listed on the periodic table Example: Example: An atom of sodium with atomic number 11 and a mass number 23 has the following atomic symbol: mass number 23 Gives the mass of “average” atom of each element compared to 12C protons + neutrons Average atom based on all the isotopes and their abundance % Na atomic number Na 22.99 Atomic mass (!unlike mass #!) is not a whole # 11 3 4 1 Why Do We Care About Isotopes? Answer: Nuclear Stability! Radiation Primer: Subatomic Particles Revisited Radiation comes from nucleus of an atom Unstable nucleus emits a particle or energy “belt of stability” protons and/or neutrons typically lost from nucleus changing identity of element Unstable nuclei α alpha (particle) Radioactive Can decay to release alpha particles, beta particles and positrons. β beta (particle) γ gamma (pure energy) 5 6 Half-Life of a Radioisotope Examples of Half-Life Isotope C-15 RaRa-224 RaRa-223 I-125 C-14 U-235 The time for the radiation level to fall (decay) to oneone-half its initial value decay curve initial 1 half-life 8 mg 4 mg 2 2 mg 3 Half life 2.4 sec 3.6 days 12 days 60 days 5700 years 710 000 000 years 1 mg 7 8 2 Medical Uses of Radiation Diagnostic Methods Radioisotopes behave chemically the same as stable isotopes of the same atom Thus, can use to target an organ or a physiological process Usually γ-emitters because radiation has to emerge from the body if the imaging equipment is to see it Nuclear medicine has two main arenas Diagnostic methods Small amounts of radioisotopes administered to help image an organ or follow a physiological process Therapeutic methods Larger radiation doses to deliver fatal punch to diseased tissue 9 PET Scans Some Radio-Imaging Isotopes P-32 CrCr-51 FeFe-59 SeSe-75 I-131 HgHg-197 10 Positron Emission Tomography Good for following physiological processes Patient given shortshort-lived β+-emitter Eye tumors Spleen shape and GI disorders Bone marrow function Pancreas scan Thyroid malfunction Kidney scan 11 6 - β+ C 11 5 B + 0 1 e+ t1/2 20 min β+ The β hits an electron in tissues 11 12 3 PET Scans A PET scan instrument places patient inside a ring of detectors to see the paired, 180o gamma rays With annihilation event, a pair of gamma rays are emitted in opposite directions 0 1 e+ + 0 -1 e- 2γ Very clear images because signal distinguished from background radiation (only seen in one direction) 13 14 http://www.breastcancer.org/testing_pet.html PET Scans Can incorporate CC-11 into a number of organic compounds to follow Blood flow Glucose metabolism Oxygen uptake Find brain areas associated with epilepsy Find hard to spot tumors Abnormal lymph nodes imaged by PET scan 15 Also: http://www.ldcmri.com/html/pet_scans.html 16 4 Therapeutic Radiation Therapeutic Radiation Can also deliver a radiation dose with an internal administration of selected isotopes I-131 targets thyroid cancer - thyroxine contains iodine I-125 crystals implanted in prostate gland to deliver continuous radiation (t1/2 = 60 days) Y-90 implanted in pituitary to slow tumor growth everywhere Selective destruction of pathological cells and tissues Rapidly dividing cells most vulnerable thus it targets cancer Used when cancer is not well localized CoCo-60 is common source of xx-rays and γ-rays 17 18 Radio-Tracer Compounds Huge use of radioisotopes to study chemical reactions Can see 10-19g/L - almost individual atoms Photosynthesis - carbon in glucose comes from CO2 Calcium - uptake is 90% efficient in children; 40% efficient in adults Zinc - uptake by trees in winter 2 ft/day Shifting Gears… Electron Energy Levels 19 20 5 Atomic Spectra Characteristics of Electrons Atoms can absorb and emit radiation Absorption Usually, a source of white light passes through a sample, and the atoms absorb only specific frequencies of light. Emission When excited atoms emit photons, the frequencies of the photon are specific. specific. “quantization of energy” Extremely small mass Located outside the nucleus Moving at extremely high speeds roughly in a sphere Form the “glue” that holds compounds together Have specific energy levels COOL DEMO… 21 22 ABSORPTION EMISSION 23 A ball on a staircase shows some properties of quantized energy states. 24 6 Explanation for Discrete Energies: the Bohr Model Line Spectra & the Bohr Model First model of the electron structure of atoms Gives levels where an electron is most likely to be found Incorrect today, but a key in understanding the atom Bohr Model • Bohr noted the line spectra of certain elements and assumed the electrons were confined to specific energy states. These were called orbits. 25 Quantum Mechanics 26 Electron Levels (Shells) Describes the arrangement of electrons in atoms in terms of: Main or principal energy levels (n) Can describe electrons with “quantum numbers” Energy subshells Orbitals (space occupied within the atom) 27 Contain electrons that are similar in energy and distance from nucleus Low energy electrons are closest to the nucleus Identify by numbers 1, 2, 3, 4, 5, 6….. The first shell (1) is lowest in energy, 2nd level next and so on 1<2<3<4 28 7 Order of Electron Filling Number of Electrons All electrons in the same energy level have similar (BUT NOT IDENTICAL) energy. Shell 1 2 electrons Shell 2 8 electrons Shell 3 18 electrons (8 first,10 later) Maximum number of electrons in any electron level = 2n2 n =1 n =2 n =3 2(1 2(1)2 2(2 2(2)2 2(3 2(3)2 = = = 2 8 18 Order of filling for the first 20 electrons Shell 1 2e 2 8e 3 8e 4 2e 29 Electron Configuration Lists the shells containing electrons Written in order of increasing energy Element Shell 1 2 He 2 C 2 4 F 2 7 Ne 2 8 Al 2 8 Cl 2 8 30 Learning Check A. The electron configuration for sulfur 1) 2,6 2) 8,2,6 3) 2, 8, 6 3 B. The element in period 3 with two electrons in the outermost energy level 1) Mg 2) Ca 3) Be 3 7 31 32 8 Orbitals: “Locations” of the Electrons s Orbitals An s orbital • has a spherical shape around the nucleus. • is found in each energy level. • threethree-dimensional spaces around a nucleus where an electron is most likely to be found. • have shapes that represent electron density (not a path the electron follows). follows). n=3 n=2 • each orbital can hold up to 2 electrons. n=1 Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings 33 34 p Orbitals Electrons in Energy Levels n = 11- 4 A p orbital • has a twotwo-lobed shape. • is one of three p orbitals in each energy level from n = 2. Energy Level 1 2 3 4 Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings 35 Orbitals 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f Maximum No. of Electrons 2 2 6 2 6 10 2 6 10 14 Total Electrons 2 8 18 32 36 9 Organization of the Periodic Table s1 s2 1 2 3 4 5 6 Periodic Law All the elements in a group have the same electron configuration in their outermost shells Outermost electrons are called valence electrons Elements with same # of valence electrons display similar chemical & physical properties!!! p1 p2 p3 p4 p5 p6 d1 - d10 Example: Group 2 Be 2, 2 Mg 2, 8, 2 Ca 2, 2, 8, 2 f1 - f14 37 38 Periodic Table Groups and Periods 39 Note: Two methods for numbering; we will use 1A, 2A, etc. 40 10 Metals, Nonmetals, and Metalloids The heavy zigzag line separates metals and nonmetals. • Metals are located to the left. • Nonmetals are located to the right. • Metalloids are located along the heavy zigzag line between the metals and nonmetals. Metals, Nonmetals, & Metalloids Metals • shiny and ductile • good conductors of heat and electricity Nonmetals • dull, brittle, and poor conductors • good insulators Metalloids • better conductors than nonmetals, but not as good as metals • used as semiconductors and insulators Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings 41 More Periodic Trends 42 Atomic Radius Within A Group How Atomic Orbital filling affects: Atomic Size (Radius) Ionization Energy Atomic radius increases going down each group of representative elements. Definitions…. Atomic Radii: Distance between center of nucleus and outer electron shell Ionization Energy : Cost of removing an efrom a neutral atom Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings 43 44 11 Atomic Radius Across a Period Ionization Energy In a Group Going across a period left to right, • an increase in number of protons increases attraction for valence electrons. • atomic radius decreases. decreases. Going up a group of representative elements, • the distance decreases between nucleus and valence electrons. • the ionization energy increases. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings 45 46 Ionization Energy • Metals have lower ionization energies. • Nonmetals have higher ionization energies. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings 47 12