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10/8/13 Atomic Structure Matter Consists of Particles Aristotle John Dalton 1803 I am a genius! Democritus What is an atom? Atomic Theory of Matter based on the following postulates: 1) Each element is composed of particles called atoms. 2) All atoms of a given element are identical. 3) Atoms are neither created nor destroyed in any chemical reaction. 4) A given compound always has the same relative numbers and kinds of atoms. JJ Thompson (1897) ! Smallest particle of an element that retains the properties of that element Gold Atom Model Robert Millikan (1909) ! Used a cathode ray tube ! Oil drop experiment and found negatively charged particles ! Discovered electrons (e) ! Plum pudding model ! Could not determine the mass ! Measured charge of electron n Charge = -1 ! Calculated mass of electron Mass = 9 x 10-28 grams or…. n 1/2000 mass of H atom n 1 10/8/13 Gold Foil Experiment Ernest Rutherford (1909) ! Discovered: n 1. nucleus is positively charged, very dense, & very small (1/10,000 of diameter) n 2. electrons are in space surrounding nucleus n 3. most of atom is empty space Scientists Recap What does this mean? Aristotle Democritus •Believed universe made of 4 elements: earth, air, fire, and water •Believed matter made of particles he called atoms Dalton •Atomic Theory of Matter Thompson •Discovered electrons using cathode ray tube •Plum pudding model Millikan •Oil drop experiment Rutherford •Gold foil experiment •Nucleus positive and dense •Measured mass and charge of electron Thompson’s Plum Pudding model is now replaced with Rutherford’s model. Modern Atomic Theory •Electrons in space surrounding nucleus Old vs. New ! Electrons (e) occupy “cloud” outside of nucleus n They DO NOT orbit around the nucleus like the planets do around the sun… n Impossible to know where an electron is at any given time Orbit Electron Cloud 2 10/8/13 Protons (P) Neutron (N) ! No charge = 0 ! Proton ! Mass = 1.675 x 10-23 g or 1 n +1 charge n Mass: 1.673 x 10-23 g (almost mass of H atom) or 1 amu n Location: nucleus amu ! Location: nucleus ! Discovered by Chadwick (1932) ! Neutron is slightly heavier than proton (1.675 vs. 1.673 x 10-23 grams) Atomic Number 11 Na Atomic number will be the smallest number shown! 22.99 ! Atomic # = number of protons in an atom 11 element have the same ! All atoms of given atomic number ! Atoms are neutral therefore…. Na n Positive charge = negative charge OR 22.99 n Number of protons = number of electrons Mass Number ! Mass number = number of protons & neutrons Neutron Electron Nucleus Mass number = 12 Proton Atomic number = 6 Element = Carbon (C) Note: Use the mass number on the periodic table, unless I tell you otherwise. Isotopes Isotopes of Carbon ! Atoms of the same element having different number of neutrons SAME: Element # Neutrons = mass number – atomic number C-14 or DIFFERENT: # Neutrons #P #e C-12 Masses 12 6 C Nucleus or 6P 6e 6N 14 6 C 6P 6e 8N Nucleus 3 10/8/13 Isotopes of Hydrogen Hydrogen – 1 (protium) 1P 1 H 1e 1 0N Hydrogen – 2 (deuterium) 1P 2 H 1e 1 1N “Heavy Water” Hydrogen – 3 (tritium) 1P 3 H 1e 1 2N Radioactive Atomic Mass Let’s try it. ! Atomic mass on the Periodic Table is the ! Mass of atom relative to Carbon-12 (standard) ! Unit = atomic mass unit or amu n 1 amu = 1/12 mass of a C-12 atom Why are they fractional? ! Why is B 10.81 amu and not just 11? ! There are two isotopes of B 10 11 B B 5 5 ! Scientists take an average of the isotopes to calculate the atomic mass ! Which one is more abundant in nature? average atomic mass, based on abundances of each isotope in nature. ! What is the atomic mass of Li? 6.94 amu ! What is the atomic mass of Cl? 35.45 amu ! What is the atomic mass of As 74.92 amu Calculating the average atomic mass Cl: 75.5% is Cl-35 (atomic mass = 34.97 amu) 24.5% is Cl-37 (atomic mass = 36.97 amu) What is the atomic mass on the Periodic Table? .755(34.97) + .245(36.97) = 35.46 = 35.5 amu 4 10/8/13 Put it all together I need some volunteers! Model of the atom 19 F 9 Draw the model on the board. Electron Cloud 9P 10 N Nucleus 7 Li 3 39 K 19 16 O 8 23 Na 11 # Neutrons = atomic mass – atomic number Nuclear Reactions Stable Nuclei ! The composition of the nucleus is changed. ! Stable nuclei are NOT radioactive ! Stable nuclei are elements #1-83 (#84àare radioactive) ! Strong nuclear forces = attraction between particles in nucleus that hold it together n P P VERY STRONG! t, (= 3 Transparency Master Types of Radioactive Decay 130 120 • #1-20 equal number of protons & neutrons for stable nuclei • #21-83 nuclei need more & more neutrons to be stable • #84à radioactive (all isotopes 110 100 90 {\-- a' c BO f 70 o L o c o L o _o 60 E z-, 50 40 30 ! 1. Alpha particles ! 2. Beta particles ! 3. Gamma particles 20 10 0 10 20 30 40 50 60 70 80 Number of protons @ Prentice-Hall, lnc Chapter 3 7 5 10/8/13 Alpha Particles 4 = α 2 He 2 4 ! Consists of 2 protons Write the nuclear equation for the alpha 231 decay of Pa & 2 neutrons ! Has a +2 charge ! Identical to a He-4 nucleus ! Stopped by paper 226 88 Alpha decay problems 91 231 227 Pa à 42 α + 89 Ac 91 Write the nuclear equation for the alpha decay of 244 Pu 4 222 244 2 86 94 Ra à α + Rn Beta particles 0 -1 0 β =-1 e 94 Pu 240 4 à α + U 92 2 Beta Decay problems Write a nuclear equation for the beta decay of 223 ! High speed electron is emitted out from atom ! -1 charge ! Stopped by heavy clothing ! Neutron changes into a proton & an electron 131 53 Fr 87 223 87 Fr à 0 -1 e + Ra 223 88 Write a nuclear equation for the beta decay of 50 22 Ti 0 131 50 -1 54 22 I à e + Xe Gamma Radiation 0 0 Ti à 0 -1 50 e+ V 23 Recap ! High energy ! Radiant energy ! 0 charge, 0 mass ! Most penetrating n Stopped by lead or concrete 6 10/8/13 Other Nuclear Reactions ! Fission is splitting of the nucleus Fission Chain Reaction ! Fusion is joining of nuclei Fusion The Mole The Mole What is a mole really? ! 1 mole contains 6.02 x 1023 atoms ! The atomic mass of an element expressed in grams is: n 1 mole of that substance OR its gram-atomic mass (GAM) 7 10/8/13 Lets try it. Put it together 16.00 g of O = 1 mol of O atoms and contains 6.02 x 1023 atoms 32.06 g of S = 1 mol of S atoms and contains _____ 6.02 x 1023atoms. _________ 1 mole = GAM (from P.T.) and 1 mole = 6.02 x 1023 particles _____ 24.31 g of Mg = 1mol of Mg atoms and contains 6.02 x 1023 atoms. _________ Avogadro’s Number Mole conversions What is the mass in grams of 4.00 moles of krypton (Kr) atoms? 4.00 moles Kr x 83.80 g Kr = 335.2 = 335 g Kr 1 mole Kr Practice What is the mass of 0.30 moles of sulfur (S)? 0.30 mol S x 32.06 g S = 9.618 = 9.6 g of S 1 mol S 83.80 g Kr = 1 mole Kr 32.06 g S = 1 mole S From Periodic Table More practice How many moles of boron (B) are present in 22 grams? 22 g B x 1 mol B = 2.035 = 2.0 mol of B 10.81 g B 10.81 g B = 1 mole B Again How many moles are there in 9.3 g of phosphorous (P)? 9.3 g P x 1 mole P = .30029= .30 mol of P 30.97 g P 1 mole P = 30.97 g P 8 10/8/13 The mole map 6.02 x 1023 atoms 1 mol Find the number of moles in 22.5 g of beryllium (Be). 1 mol # particles (atoms) 6.02 x 1023 atoms GAM 1 mol 22.5 g Be x 1 mol Be = 2.4972 = 2.50 mol of Be 9.01 g Be MOLE GAM 1 mol Last time 1 mole Be = 9.01 g Be g MOLE ! A mole (mol) is defined as the number of STOP! The Mole Start of Chem Calculations: atoms in exactly 12 grams of Carbon-12. ! The mole is the SI unit for the amount of a substance. Mol ! The Carbon-12 based Atomic Mass Scale- By definition, an atom of this isotope is defined as having the mass of exactly 12.000 amu (atomic mass units) . In other words, an amu is defined as 1/12th of the mass of one atom of Carbon-12. Why Carbon-12? Carbon is a very common element, available to any scientist and by choosing the amu to be this size, the atomic masses of nearly all the other elements are almost whole numbers, with the lightest atom having a mass of ~1. Hydrogen-1= 1.007825amu when Carbon is assigned a mass of exactly 12. ! Molar Mass [g/mol]= mass of one mole in grams to two decimal places off the periodic table. MOL Molecules ! 1 mol=1GAM=6.02 x1023molecules ! 6.02 x 1023molecules - Avogadro’s # ! Avogadro’s number: is a defined mass of an element (it’s atomic weight) there is a precise number of atoms. ! Grams 9 10/8/13 CONVERSIONS-dimensional Analysis (again!) ! Mole: amount of substance in grams PRACTICE: ! COMPLETE FORMULA WS ! COMPLETE PROBLEM WS Determine which of Dalton’s postulates explains each observation? 1. Matter can never really be thrown away. That is one reason that recycling is important 2. The formula for ethanol is C2H6O, and the formula for acetic acid in vinegar is C2H4O2 3. There is no difference between Cu found in an ancient Mayan necklace and Cu wire freshly made from Cu ore. 4. Zn is a metal that is softer that Fe and it reacts more readily with acid than Fe does. 5. When Methane, CH4, Burns, it combines with O2, in the air to form molecules of H2O and CO2. JJ Thompson (1897) I. Law of Conservation of Matter: Discovered by:? II. Law of Constant Composition: Joseph Proust A given compound always contains the same elements in the same proportions by mass Water= 11% H2 , 89%O2 Iron Oxide= FeO= 25% O2, 75%Fe CATHODE RAY TUBE • JJ Thompson conducted a series of systemic studies on cathode rays • Watched the deflection of charges in a CRT and put forth the idea that Atoms were composed of (+) and (-) charges. • Negative charges = electrons • Electron mass ratio= 1.76 x 108 C/g COULD NOT DETERMINE MASS! Electrical current: moving stream of electrical charge Cathode: negative electrode Anode: positive electrode 10 10/8/13 JJ Thompson “Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen?" -- J.J. Thomson. Michael Faraday Suggested that the structure of atom was somewhat related to electricity. Atoms contain particles that have electrical charge. Story of electricity and the atom includes a certain American whose name you should find very familiar? • Cathode Ray Tube (CRT) • Discovered the electron Benjamin Franklin Made distinction between 2 kinds of electrical charge, Positive (+) and negative (-) Opposite charges: attract. Like Charges: repel Where do + and – charges come from? What are their physical properties? Led to his discovery of another Electrical device called a battery. Question: Radioactivity ! How did Rutherford use the results of his ! Henri Becquerel (1896) alpha scattering experiment experiment to challenge the plum pudding model of the atom? ! How does Rutherford’s model of the atom differ from the plum pudding model? • Discovered that U exhibits radioactivity (spontaneous emission of radiation from an element) • Uranium one of many naturally radioactive elements 11 10/8/13 Radioactivity Radioactivity Radioactivity: spontaneous release of energetic particles or photons from the nuclei. Original nucleus decomposes, decays to form a new nucleus, releasing radiation in the process. ! There are 3 types of radioactive decay. 1.Alpha 2.Beta 3.Gamma ! Radioactive decay- when an atom emits α,β,γ radiation. ! Ernest Rutherford ! ! Marie and Pierre Curie - Isolated 2 other radioactive elements: Polonium and Radium Alpha decay Student of JJ Thompson ! Alpha decay: a nucleus emits an alpha particle consisting ! Gold Foil Experiment (α scattering expt) of 2 neutrons and 2 protons, which is a helium nucleus. ! Began in depth study of radioactivity! Found that U emits two forms of radiation and developed his nuclear model of atom. Alpha and beta decay practice problems: ! NUCLEAR EQUATION- to keep track of the reaction components Alpha decay Example: 226 222 Ra Rn + 88 86 4 He 2 Nucleus rejects a helium nucleus or alpha particle and becomes A smaller nucleus with less positive charge Beta decay ! Beta decay: a neutron in a nucleus spontaneously decays into a proton, an electron, and a neutrino, thus creating a different element. Carbon-14 > nitrogen-14. Due to a weak force. ! Stream of high speed electrons- not electrons in motion around nucleus ! Beta radiation comes from changes in nucleus: nuetrons changes into a proton and a electron, proton remains in nucleus, electron (β particle) propelled out of nucleus at high speeds. ! Mass # is zero ! 100x more penetrating that alpha radiation ! Able to pass through clothing and damage skin 12 10/8/13 Beta Decay Problems Gamma decay ! Gamma radiation: nuclear particles shift energy levels, like ! Beta Decay Example: 131 I 53 131 54 Xe + 0 -1 β ! ! Turns a neutron in the nucleus into a proton, ejecting a beta Particle (-) or electron in the process. ! ! electrons undergoing a quantum leap, and X-rays or gamma rays are emitted. Very dangerous does not consist of particles Penetrates solid material including body tissues Stopped by lead or concrete In Radioactive Decay- sums of mass # and Atomic # are the same before and after the rxn. Radiation is dangerous because it strips away the electrons from atoms in cells, causing them to malfunction. Radiation Experiment ! (+) (-) Half Life- Half Life Contents [hide] ! Half-life: the amount of time it takes half of a batch of radioactive material to decay. Ranges from less than a second to billions of years, depending upon the isotope. ! Radiometric dating: The technique of measuring quantities of isotopes in order to determine time scales. e.g.: the ratio of radioactive carbon-14 to carbon-13 in an organism at the time of its death is the same as the general environment. After death, no more carbon is taken in, so the % of C-14 goes down as it decays into N-14. Half decays every 5700 years, so time of death can be calculated. ! Eg the Shroud of Turin and the Dead Sea Scrolls were dated at the University of Arizona with this technique. ! EXTRA CREDIT ON THE TEST: LOOK UP HOW RADIOACTIVE DATING WAS USED FOR THE KENNEWICK MAN AND THE Haraldskセr Woman 13