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Atomic Structure Matter Consists of Particles Aristotle Democritus John Dalton 1803 I am a genius! 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. What is an atom? Smallest particle of an element that retains the properties of that element Gold Atom Model JJ Thompson (1897) Used a cathode ray tube and found negatively charged particles Discovered electrons (e) Plum pudding model Could not determine the mass Robert Millikan (1909) Oil drop experiment Measured charge of electron Charge = -1 Calculated mass of electron Mass = 9 x 10-28 grams or…. 1/2000 mass of H atom Gold Foil Experiment Ernest Rutherford (1909) Discovered: 1. nucleus is positively charged, very dense, & very small (1/10,000 of diameter) 2. electrons are in space surrounding nucleus 3. most of atom is empty space What does this mean? Thompson’s Plum Pudding model is now replaced with Rutherford’s model. Scientists Recap Aristotle •Believed universe made of 4 elements: earth, air, fire, and water Democritus •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 •Measured mass and charge of electron Rutherford •Gold foil experiment •Nucleus positive and dense •Electrons in space surrounding nucleus Modern Atomic Theory Electrons (e) occupy “cloud” outside of nucleus They DO NOT orbit around the nucleus like the planets do around the sun… Impossible to know where an electron is at any given time Old vs. Orbit New Electron Cloud Protons (P) Proton +1 charge Mass: 1.673 x 10-23 g (almost mass of H atom) or 1 amu Location: nucleus Neutron (N) No charge = 0 Mass = 1.675 x 10-23 g or 1 amu Location: nucleus Discovered by Chadwick (1932) Neutron is slightly heavier than proton (1.675 vs. 1.673 x 10-23 grams) 11 Atomic Number 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 Positive charge = negative charge OR 22.99 Number of protons = number of electrons Mass Number Mass number = number of protons & neutrons Neutron Electron Nucleus Mass number = 12 Atomic number = 6 Element = Carbon (C) Proton Isotopes Atoms of the same element having different number of neutrons SAME: Element DIFFERENT: # Neutrons #P #e # Neutrons = mass number – atomic number Masses Note: Use the mass number on the periodic table, unless I tell you otherwise. Isotopes of Carbon C-12 C-14 or or 12 6 C Nucleus 6P 6e 6N 14 6 C 6P 6e 8N Nucleus Isotopes of Hydrogen Hydrogen – 1 (protium) 1P 1 H 1e 1 0N Hydrogen – 2 (deuterium) 1P 2 H 1e 1 1N Hydrogen – 3 (tritium) 1P 3 H 1e 1 2N Radioactive “Heavy Water” Atomic Mass Mass of atom relative to Carbon-12 (standard) Unit = atomic mass unit or amu 1 amu = 1/12 mass of a C-12 atom Let’s try it. Atomic mass on the Periodic Table is the 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 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? *NOTE Atomic masses are generally fractional Mass numbers are rounded to the nearest whole number. For example: Carbon’s atomic mass is 12.01 Carbon’s mass number is 12 What about Be, Na, B? 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 Put it all together Model of the atom 19 F 9 9P 10 N Electron Cloud Nucleus # Neutrons = atomic mass – atomic number I need some volunteers! Draw the model on the board. 7 Li 3 39 K 19 16 O 8 23 Na 11 Nuclear Reactions The composition of the nucleus is changed. Stable Nuclei Stable nuclei are NOT radioactive Stable nuclei are elements #1-83 (#84are radioactive) Strong nuclear forces = attraction between particles in nucleus that hold it together VERY STRONG! P P • #1-20 equal number of protons & neutrons for stable nuclei • #21-83 nuclei need more & more neutrons to be stable • #84 radioactive (all isotopes Types of Radioactive Decay 1. Alpha particles 2. Beta particles 3. Gamma particles = α 2 4 Alpha Particles Consists of 2 protons & 2 neutrons Has a +2 charge Identical to a He-4 nucleus Stopped by paper 226 88 Ra α + Rn 4 2 222 86 4 2 He Alpha decay problems Write the nuclear equation for the alpha 231 decay of Pa 91 Pa 91 231 4 2 α 227 89 + Ac Write the nuclear equation for the alpha decay of 244 Pu 94 244 Pu 94 α 4 2 + 240 92 U Beta particles 0 -1 0 -1 β= e High speed electron is emitted out from atom -1 charge Stopped by heavy clothing Neutron changes into a proton & an electron I e + Xe 131 53 0 -1 131 54 Beta Decay problems Write a nuclear equation for the beta decay of 223 Fr 87 223 87 Fr 0 -1 223 e + Ra 88 Write a nuclear equation for the beta decay of 50 22 Ti 50 22 Ti 0 -1 50 23 e+ V 0 Gamma Radiation High energy Radiant energy 0 charge, 0 mass Most penetrating Stopped by lead or concrete 0 Recap Other Nuclear Reactions Fission is splitting of the nucleus Fusion is joining of nuclei Fission Chain Reaction Fusion The Mole The Mole 1 mole contains 6.02 x 1023 atoms The atomic mass of an element expressed in grams is: 1 mole of that substance OR its gram-atomic mass (GAM) What is a mole really? Lets try it. 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. _________ _____ 24.31 g of Mg = 1mol of Mg atoms and contains 6.02 x 1023 atoms. _________ Put it together 1 mole = GAM (from P.T.) and 1 mole = 6.02 x 1023 particles 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 83.80 g Kr = 1 mole Kr From Periodic Table 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 32.06 g S = 1 mole S 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 Last time Find the number of moles in 22.5 g of beryllium (Be). 22.5 g Be x 1 mol Be = 2.4972 = 2.50 mol of Be 9.01 g Be 1 mole Be = 9.01 g Be The mole map 6.02 x 1023 atoms 1 mol MOLE 1 mol 6.02 x 1023 atoms GAM 1 mol 1 mol GAM g # particles (atoms) STOP! MOLE A mole (mol) is defined as the number of atoms in exactly 12 grams of Carbon-12. The mole is the SI unit for the amount of a substance. The Mole Start of Chem Calculations: The Carbon-12 based Atomic Mass ScaleBy 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. Mol Molar Mass [g/mol]= mass of one mole in grams to two decimal places off the periodic table. Grams 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. 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. 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 JJ Thompson (1897) 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 “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. JJ Thompson •Cathode Ray Tube (CRT) •Discovered the electron 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? 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: How did Rutherford use the results of his 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? Radioactivity Henri Becquerel (1896) • Discovered that U exhibits radioactivity (spontaneous emission of radiation from an element) • Uranium one of many naturally radioactive elements 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 ,, radiation. Radioactivity Marie and Pierre Curie - Isolated 2 other radioactive elements: Polonium and Radium Ernest Rutherford Student of JJ Thompson Gold Foil Experiment (α scattering expt) Began in depth study of radioactivityFound that U emits two forms of radiation and developed his nuclear model of atom. Alpha decay Alpha decay: a nucleus emits an alpha particle consisting of 2 neutrons and 2 protons, which is a helium nucleus. Alpha and beta decay practice problems: NUCLEAR EQUATION- to keep track of the reaction components Alpha decay Example: 226 222 4 Ra Rn + He 88 86 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 ( 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 Beta Decay Problems Beta Decay Example: 131 131 I Xe 53 54 + 0 -1 Turns a neutron in the nucleus into a proton, ejecting a beta Particle (-) or electron in the process. In Radioactive Decay- sums of mass # and Atomic # are the same before and after the rxn. Gamma decay Gamma radiation: nuclear particles shift energy levels, like 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 Radiation is dangerous because it strips away the electrons from atoms in cells, causing them to malfunction. Radiation Experiment (+) (-) 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 carbon14 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 Half Life-