JJ Thompson (1897) Robert Millikan (1909)

Transcription

1 Atomic Structure Matter Consists of Particles Aristotle Democritus John Dalton 183 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) Robert Millikan (199)! Used a cathode ray tube and found negatively charged particles! Discovered electrons (e)! Plum pudding model! Could not determine the mass! Oil drop experiment! Measured charge of electron n Charge = -1! Calculated mass of electron n Mass = 9 x 1-28 grams or. n 1/2 mass of H atom 1

2 Gold Foil Experiment Ernest Rutherford (199)! Discovered: n 1. nucleus is positively charged, very dense, & very small (1/1, of diameter) n 2. electrons are in space surrounding nucleus n 3. most of atom is empty space What does this mean? Thompson s Plum Pudding model is now replaced with Rutherford s model. Aristotle Democritus Dalton Thompson Millikan Rutherford Scientists Recap Believed universe made of 4 elements: earth, air, fire, and water Believed matter made of particles he called atoms Atomic Theory of Matter Discovered electrons using cathode ray tube Plum pudding model Oil drop experiment Measured mass and charge of electron Gold foil experiment Nucleus positive and dense Electrons in space surrounding nucleus Modern Atomic Theory 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

3 Protons (P)! Proton n +1 charge n Mass: x 1-23 g (almost mass of H atom) or 1 amu n Location: nucleus Neutron (N)! No charge =! Mass = x 1-23 g or 1 amu! Location: nucleus! Discovered by Chadwick (1932)! Neutron is slightly heavier than proton (1.675 vs x 1-23 grams) Atomic Number Na 11 Atomic number will be the smallest number shown!! Atomic # = number of protons in an atom! All atoms of given 11 element have the same atomic number Na! Atoms are neutral therefore. n Positive charge = negative charge OR n Number of protons = number of electrons Mass Number! Mass number = number of protons & neutrons Electron Mass number = 12 Atomic number = 6 Element = Carbon (C) Neutron Proton Nucleus Isotopes! Atoms of the same element having different number of neutrons SAME: Element # P # e DIFFERENT: # Neutrons Masses Note: Use the mass number on the periodic table, unless I tell you otherwise. Isotopes of Carbon C-12 or 12 6 C 6 P 6 e 6 N C-14 or 14 6 C 6 P 6 e 8 N # Neutrons = mass number atomic number Nucleus Nucleus 3

4 Isotopes of Hydrogen Heavy Water Hydrogen 1 Hydrogen 2 (protium) (deuterium) 1 P 1 P 1 2 H 1 e H 1 e 1 1 N 1N Hydrogen 3 (tritium) 1 P 3 H 1 e 1 2 N Radioactive Atomic Mass Let s try it.! 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! 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? amu! What is the atomic mass of As amu Why are they fractional?! Why is B 1.81 amu and not just 11?! There are two isotopes of B 1 B 5 11 B 5! Scientists take an average of the isotopes to calculate the atomic mass! Which one is more abundant in nature? Calculating the average atomic mass Cl: 75.5% is Cl-35 (atomic mass = amu) 24.5% is Cl-37 (atomic mass = amu) What is the atomic mass on the Periodic Table?.755(34.97) +.245(36.97) = = 35.5 amu 4

5 1/8/13 Put it all together I need some volunteers! Model of the atom 19 F 9 Draw the model on the board. Electron Cloud 9P 1 N Nucleus 7 Li 3 39 K 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 (#84à are 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 #1-2 equal number of protons & neutrons for stable nuclei #21-83 nuclei need more & more neutrons to be stable #84à radioactive (all isotopes {\-- a' c BO f 7 o L o c o L o _o 6 E z-, 5 4 3! 1. Alpha particles! 2. Beta particles! 3. Gamma particles Number of Prentice-Hall, lnc Chapter 3 7 5

6 Alpha Particles 4 α = He Alpha decay problems! Consists of 2 protons & 2 neutrons! Has a +2 charge! Identical to a He-4 nucleus! Stopped by paper Ra à 2 α + 86 Rn Write the nuclear equation for the alpha 231 decay of 231 Pa 91 Pa 4 91 à α Ac Write the nuclear equation for the alpha decay of Pu Pu 94 à 2 α U Beta particles! High speed electron is emitted out from atom! -1 charge! Stopped by heavy clothing! Neutron changes into a proton & an electron 131 β -1-1 = e 131 I à e + Xe Beta Decay problems Write a nuclear equation for the beta decay of 223 Fr Fr 87 à e Ra Write a nuclear equation for the beta decay of 5 22 Ti 5 5 Ti 22 à -1 e + 23 V Gamma Radiation! High energy! Radiant energy! charge, mass! Most penetrating n Stopped by lead or concrete Recap 6

7 1/8/13 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 6.2 x 123 atoms! The atomic mass of an element expressed in grams is: n 1 mole of that substance OR its gram-atomic mass (GAM) 7

8 Lets try it. Put it together 16. g of O = 1 mol of O atoms and contains 6.2 x 1 23 atoms 32.6 g of S = 1 mol of S atoms and contains 6.2 x 1 23 atoms g of Mg = 1mol of Mg atoms and contains 6.2 x 1 23 atoms. 1 mole = GAM (from P.T.) and 1 mole = 6.2 x 1 23 particles Avogadro s Number Mole conversions What is the mass in grams of 4. moles of krypton (Kr) atoms? 4. moles Kr x 83.8 g Kr = = 335 g Kr 1 mole Kr 83.8 g Kr = 1 mole Kr Practice What is the mass of.3 moles of sulfur (S)?.3 mol S x 32.6 g S = mol S = g of S 32.6 g S = 1 mole S From Periodic Table More practice How many moles of boron (B) are present in 22 grams? Again How many moles are there in 9.3 g of phosphorous (P)? 22 g B x 1 mol B 1.81 g B = 2.35 = 2. mol of B 9.3 g P x 1 mole P =.329 =.3 mol of P 3.97 g P 1.81 g B = 1 mole B 1 mole P = 3.97 g P 8

9 Last time The mole map Find the number of moles in 22.5 g of beryllium (Be) g Be x 1 mol Be 9.1 g Be = = 2.5 mol of Be 1 mol GAM MOLE GAM 1 mol 6.2 x 1 23 atoms 1 mol 1 mol 6.2 x 1 23 atoms # particles (atoms) 1 mole Be = 9.1 g Be g MOLE STOP!! 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 Scale- By definition, an atom of this isotope is defined as having the mass of exactly 12. 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= amu 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=6.2 x1 23 molecules! 6.2 x 1 23 molecules - Avogadro s #! Avogadro s number: is a defined mass of an element (it s atomic weight) there is a precise number of atoms. 9

10 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 C 2 H 6 O, and the formula for acetic acid in vinegar is C 2 H 4 O 2 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, CH 4, Burns, it combines with O 2, in the air to form molecules of H 2 O and CO 2. 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% H 2, 89%O 2 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 1 8 C/g COULD NOT DETERMINE MASS! Electrical current: moving stream of electrical charge Cathode: negative electrode Anode: positive electrode 1

11 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. Cathode Ray Tube (CRT) Discovered the electron Michael Faraday Benjamin Franklin 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? 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 11

12 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.! 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 radioactivity-! Found 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: Ra Rn + He 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! 1x more penetrating that alpha radiation! Able to pass through clothing and damage skin 12

13 Beta Decay Problems! Beta Decay Example: I Xe + β 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 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 57 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 r Woman Half Life- 13