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Chapter 2.1-2.3: BASIC Atomic Structure What is an “atom”? • The word atom comes from the Greek and means “indivisible”. • The smallest particle that retains the identity of the element in a chemical reaction. Early Atomic Theories 1. Democritus NO experimental evidence, but the first one to come up with the IDEA of an atom (~400 BC). • Matter is composed of tiny particles called atoms, which are indivisible, indestructible fundamental units of matter • The universe is made of: a) ATOMS b) EMPTY SPACE Early Atomic Theories 2. Aristotle & Plato (~400 BC) -opposed Democritus; wrong, but more popular… believed matter to be continuous and composed of 4 elements….root of the Medieval understanding of humors (e.g. phlegmatic, sanguine, etc.) and the practice of bleeding… •Earth-cool, heavy •Wind-light •Fire-hot •Water-wet The 4 elements of “Hyle” Alchemists • Experimented • Recorded observations!!! • IMPORTANT: They developed systematic procedures and methods of recording data Contributors to Dalton’s Atomic Theory 1. Antoine Lavoisier (France 1782) -Law of Conservation of Mass: In a chemical reaction, mass is conserved. 2. Joseph Proust (France 1799) -Law of Definite Proportions: The elements that comprise a compound are always in a certain proportion by mass. Example (Law of Definite Proportion): • KCl always contains potassium and chlorine in a ratio of “39.09 to 35.45” or “1.1 to 1” by mass. John Dalton (England 1766-1844) • Formulated 1st modern Atomic Theory (1808) • School teacher, studied works of Lavoisier & Proust • Formulated the Law of Multiple Proportions Law of Multiple Proportions (John Dalton) • When two elements combine to form more than one compound, the ratios of the mass of one element in the first compound to its mass in the second compound, (as it combines with the same mass of the other element), can always be expressed as ratios of small whole numbers( ex: 1 / 3). Example of Law of Multiple Proportions • Carbon combines with oxygen to form CO and CO2 . Mass of Carbon(g) CO 12.01 Mass of Ratio of O Oxygen(g) in CO2 to O in CO 16.00 CO2 12.01 32.00 2/1 Dalton’s Atomic Theory 1.All matter is made of tiny indivisible particles called atoms, and atoms are the building blocks of elements 2.All atoms of the an element are the same; Different elements have different atoms. 11 Dalton’s Atomic Theory 3. LAW OF MASS CONSERVATION: Atoms are not created or destroyed in chemical reactions (just rearranged when bonds are broken or formed). The total mass of the universe if ALWAYS the same…. 4. LAW OF DEFINITE PROPORTIONS: Two or more different atoms bond together in SIMPLE WHOLE NUMBER RATIOS to form a specific compound. -The ratio of elements (or the mass ratio of elements) in a particular compound is ALWAYS the same. -If the ratio changes, it’s a different compound. 12 MASS CONSERVATION Problem: • You heat 2.35 grams of metallic mercury in air, which produces 2.73 grams of a red-orange residue. Assume that the chemical change is the reaction of the metals with oxygen in air. What is the mass of the oxygen that reacts? When you strongly heat the red-orange residue, it decomposes to give back the mercury and release the oxygen, which you collect. What is the mass of oxygen you collect? MASS CONSERVATION Problem: • You place 1.85 grams of wood in a vessel with 9.45 grams of air and seal the vessel. Then you heat the vessel strongly so that the wood burns. In burning, the wood yields ash and gases. After the experiment, you weight the ash and find that its mass is 0.28 grams. What is the mass of the gases in the vessel at the end of the experiment? Law of Definite Proportions A specific compound ALWAYS has the same ratio of elements ex: H2O ex: H2O2 1. Can be expressed in terms of an ATOMIC or MOLE RATIO: – – 2. Water is always 2 H: 1 O Hydrogen peroxide is always 2 H: 2 O (or reduced, 1 H :1 O) Can be expressed in terms of a MASS RATIO: – Water is always 2 g H: 16 g O (or reduced, 1 g H : 8 g O) – Hydrogen peroxide is always 2 g H: 32 g O (or reduced, 1 g H: 16 g O) 3. Can be expressed in terms of a MASS PERCENTAGE: – – Water is always 11% H & 88.9% O by mass Hydrogen peroxide is always 5.9% H & 94.1% O 2.0 g H/ 18.0g H2O x 100% = 11 % H 16.0 g O/ 18.0g H2O x 100% = 88.9 % O 2.0 g H/ 34.0g H2O2 x 100% = 5.9 % H 32.0 g O/ 34.0g H2O2 x 100% = 92.1 % O DEFINITE PROPORTION Problem: • Aluminum metal reacts with bromine, a red-brown liquid with a noxious odor. The reaction is vigorous and produces aluminum bromide, a white crystalline substance. A sample of 27.0g of aluminum yields 266.7 g of aluminum bromide. How many grams of bromine react with 15.0 g of aluminum? Dalton’s Atomic Model Atoms have no net electrical charge. Their charge is 0 (neutral). Dalton’s Modern Atomic Theory Dalton’s theory: 1. INDIVISIBLE atom 2. IDENTICAL atoms for an element 3. LAW of MASS CONSERVATION Modern refinement: 1. Atoms can be further divided into their sub-atomic particles (electrons, protons, & neutrons) 2. ISOTOPES (atoms of an element can have different number of neutrons/ be heavier) 3. Nuclear chemistry Law of Conserviation of Mass & ENERGY; atoms of one element can be transmuted into atoms of another element in a nuclear reaction Amadeo Avogadro’s Hypothesis (Italy 1811) • Equal volumes of different gases at the same conditions (T and P) contain equal number of particles. The Size of the Atom • If you placed 100,000,000 Cu atoms side by side they would form a line only 1 cm long. • Radius of most atoms is about 5x10-11 to 2x10-10m. How did we go from Dalton’s model… to a model with subatomic particles? Subatomic Structure What’s inside an atom & how is it arranged? Discoveries that led to a refinement of Dalton’s simple atomic theory…. (OMG—there’s something inside of an atom!) Discovery of the electron THE CATHODE RAY TUBE J(oseph) J(ohn) Thompson, discoverer of the eNobel Prize, 1906 J. J. Thomson - English physicist. -Made a piece of equipment called a cathode ray tube (CRT). -A CRT is a vacuum tube - all the air has been pumped out. Application of the CRT QUESTIONS TO RESOLVE: 1. How did he detect this ray? 2. What is the charge of this ray? 3. Where does the ray originate? 4. Is the ray pure energy or is it composed of matter (something with mass and volume)? Cathode Rays—old school computer monitors & TVs Thomson’s Experiment Voltage source - + Vacuum tube Metal Disks Thomson’s Experiment Voltage source - + Thomson’s Experiment Voltage source - + Thomson’s Experiment Voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end Cathode Ray Thomson’s Experiment Voltage source • By adding an electric field Thomson’s Experiment Voltage source + By adding an electric field Thomson’s Experiment Voltage source + By adding an electric field Thomson’s Experiment Voltage source + By adding an electric field Thomson’s Experiment Voltage source + By adding an electric field Thomson’s Experiment Voltage source + By adding an electric field Thomson’s Experiment Voltage source + By adding an electric field he found that the moving pieces were negative Thomson’s Experiment • Used many different metals and gases • Beam was always the same • By the amount it bent he could find the CHARGE TO MASS RATIO. • Conclusion: • Every kind of atom had the same negative particles Thomson • Thomson also was able to estimate that the mass of the electron was equal to about 1/1840 of the mass of a hydrogen atom. -28 Mass of e- = 9.11 x 10 g • His discovery of the electron won the Nobel Prize in 1906!!! Robert Millikan • American (1869-1925) • Oil Droplet Experiment Millikan’s Experiment Atomizer + - Oil Microscope Metal Plates Millikan’s Experiment Atomizer Oil droplets + - Oil Microscope Millikan’s Experiment X-rays X-rays give some drops a charge by knocking off electrons Millikan’s Experiment - - - - - - - + Some drops would hover + + + + + + + Millikan’s Experiment - - + + From the mass of the drop and the charge on the plates, he calculated the actual charge on an elect • Link to Video: Millikan’s Oil Drop Experiment • http://www.youtube.com/watch?v=XMfYHag7 Liw Millikan’s Oil Drop Experiment + e He found the charge of the electron e = 1.602 x 10-19 C (Coulombs), by noting that the droplets always carried whole number multiples of that number. Charge of the Electron • Charge of Electron 1.6 x 10-19 C (coulombs) • Mass of Electron 9.11 x 10-28 g Electron charge, e = the basic quantity of charge. • Electric charges always exist in whole number multiples of a single basic unit, the electron, 1e, 2e, 3e, etc. • A particle with a positive charge must be present in the atom to balance each negatively charge electron. New discovery/ evidence New MODEL If atoms are neutral, and they contain negative electrons, there must also be a positive part to the atom MODEL: Thompson’s Plum Pudding Model (1st attempt at imagining what could sync up with the data): • Said the atom was like plum pudding • POSITIVE MATERIAL WITH EMBEDDED ELECTRONS More like cookie dough… Discovery of the NUCLEAR atom THE GOLD FOIL EXPERIMENT Ernest Rutherford, English physicist, 1910 -proves Thompson’s model erroneous, refines atomic model Lead block Rutherford’s experiment Gold Foil -Wanted to see how big atoms were • Used radioactive uranium, which releases alpha particles (i.e. positively charged helium nuclei) • Shot them at gold foil which can be made a few atoms thick Uranium Fluorescent Screen • When the alpha particles hit a fluorescent screen, it glows. He Expected…. • The alpha particles to pass through without changing direction very much Closeup view of gold atom: • In the plum pudding model, the positive charge & mass is spread out evenly, and would not be enough to stop the alpha particles He Expected…. What really happenned…. Rutherford • Expected • Found Rutherford’s Conclusions • The atom is mostely empty space. • All of the positive charge, and most of the mass of an atom are concentrated in a small core, called the nucleus. How it explains the experimental results: + Another view: Experimental evidence: Rutherford’s Gold Foil Expt. THE ATOM ISN’T DENSE. IT’S GOT A POSITIVELY CHARGED NUCLEUS (and lots of empty space), PROTON = +1.602 x 10-19 Coulombs New discovery/ evidence New MODEL Model: The “Nuclear” atomic model • Atom is mostly empty space • Small dense, positive piece at center “NUCLEUS” • Alpha particles are deflected by it if they get close enough Location/ Size of Subatomic Particles An atom is mostly EMPTY SPACE. Nucleus = the eye of the head on a dime; Electrons = 1/1000 nucleus = a pin head? Atom size = football stadium Discovery of the neutron AND, shortly after, a student of Rutherford discovers the NEUTRON! Chadwick-discover of the neutron (from study of radioactive substances), won a Nobel Prize for his discovery… paved the road for nuclear fission, etc. The Discovery of the Proton –Discovered by Eugen Goldstein (German) –He observed “Canal Rays” and found that they are composed of positive particles – protons. Henry Moseley • English physicist. • Concept of atomic number. Nuclear atom, refined • Useful for discussion of isotopes The atom is mostly empty space Two regions 1. Nucleus- protons and neutrons 2. Electron cloudregion where you might find an electron Subatomic Particles Particle Symbol Relative Mass Mass (g) Charge Relative to the Proton Electron e- 1(-1e) 1/1840 9.11 x 10-28 g p+ 1+ (+1e) 1 1.67 x 10-24g 0 1 1.67 x 10-24g Proton Neutron n0 Scanning Tunneling Microscope Scanning Tunneling Microscope Review 1. Compare/ contrast Thompson and Rutherford’s atomic models. 2. Which of these could be the charge of an object? A. B. C. D. 3. 0.80 x 10-19 C 2.0 x 10-19 C 3.2 x 10-19 C 4.0 x 10-19 C e = 1.602 x 10-19 C The magnitude of the charge on an electron was determined in the _______. A. B. C. D. cathode ray tube, by J.J. Thompson Millikan oil drop experiment Dalton atomic theory atomic theory of matter 4. Which one of the following is not true concerning cathode rays? A. B. C. D. E. 5. The gold foil experiment performed in Rutherford’s lab ________. A. B. C. D. E. 6. They originate from the negative electrode They travel in straight lines in the absence of electric or magnetic fields They impart a negative charge to metals exposed to them They are made up of electrons The characteristics of cathode rays depend on the material from which they are emitted confirmed the plum-pudding model of the atom led to the discovery of the atomic nucleus was the basis for Thomson’s model of the atom utilized the deflection of beta particles by gold foil proved the law of multiple proportions In the Rutherford nuclear-atom model: A. B. C. D. the heavy subatomic particles reside in the nucleus the principal subatomic particles all have essentially the same mass the light subatomic particles reside in the nucleus mass is spread essentially uniformly throughout the atom The Discovery of the Neutron • Chadwick bombarded alpha particles(helium nuclei) at Beryllium. • Neutrons were emitted and in turn hit parafin and ejected protons from the parafin. Discovery of the Neutron Neutrons • Neutrons have mass similar to protons. • No electrical charge. Atomic Number, Mass Number, Atomic Mass and Isotopes Atomic Number (Z): is the number of protons in the nucleus of the atom. Z=#p • The number of protons (atomic number) determine the identity of an element. Atoms have no overall electrical charge so, an atom must have as many electrons as there are protons in its nucleus. The atomic number of an element also equals the number of electrons in a neutral atom of that element. Ions • Positively charge atom (cation) – Atom lost electrons. • Negatively charged atom (anion) – Object gained electrons. Ex: Sodium • What is the atomic number of Sodium? • How many protons does sodium have? • How many electrons does sodium have? Mass Number (A): • The sum of the protons and neutrons in the nucleus. A=#p + #n Notation Other ways to write elements: Mass Number Mass Number 36 17Cl Atomic Number Cl-36 Nucleons: protons and neutrons Isotopes of an element have different mass numbers because they have different numbers of neutrons, but they have the same atomic number. Example: Isotopes of Carbon and Hydrogen Isotopes of Hydrogen protium deuteriumtritium H H Isotopes of Carbon H Atomic Mass Unit is a unit used to compare the masses of atoms and has the symbol u or amu. 1 amu or u is approximately equal to the mass of a single proton or neutron. Carbon-12 Chemists have defined the carbon-12 atom as having a mass of 12 atomic mass units. 1 u = 1/12 the mass of a Carbon-12 atom. Atomic Mass is the weighted average mass of all the naturally occurring isotopes of that element. =SUM OF (isotope % x isotope mass) Atomic Mass Calculations 1. Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu. • 63.5 amu Atomic Mass 2. Magnesium has three isotopes. 78.99% magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 25 with a mass of 25.9826 amu. What is the atomic mass of magnesium? • 24.3050 amu Atomic Mass • These masses appear on the periodic table.