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Greek Model “To understand the very large, we must understand the very small.” Democritus • Greek philosopher • Idea of ‘atomos’ – Atomos = ‘indivisible’ – ‘Atom’ is derived • No experiments to support idea Democritus’s model of atom No protons, electrons, or neutrons Solid and INDESTRUCTABLE Democritus DEMOCRITUS (400 BC) – First Atomic Hypothesis Atomos: Greek for “uncuttable”. Chop up a piece of matter until you reach the atomos. Properties of atoms: • indestructible. • changeable, however, into different forms. • an infinite number of kinds so there are an infinite number of elements. • hard substances have rough, prickly atoms that stick together. • liquids have round, smooth atoms that slide over one another. • smell is caused by atoms interacting with the nose – rough atoms hurt. • sleep is caused by atoms escaping the brain. • death – too many escaped or didn’t return. • the heart is the center of anger. • the brain is the center of thought. • the liver is the seat of desire. “Nothing exists but atoms and space, all else is opinion”. Four Element Theory FIRE • Aristotle was an atomist • Thought all matter was composed of 4 elements: – – – – – Earth (cool, heavy) Water (wet) Fire (hot) Air (light) Ether (close to heaven) Hot Dry ‘MATTER’ AIR Wet EARTH Cold WATER Relation of the four elements and the four qualities Blend these “elements” in different proportions to get all substances Foundations of Atomic Theory Law of Conservation of Mass Mass is neither destroyed nor created during ordinary chemical reactions. Law of Definite Proportions The fact that a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound. Law of Multiple Proportions If two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first elements is always a ratio of small whole numbers. Legos are Similar to Atoms H H2 H H O + H2 H H O2 H O H 2O H O O H H 2O Lego's can be taken apart and built into many different things. Atoms can be rearranged into different substances. Law of Multiple Proportions John Dalton (1766 – 1844) If two elements form more than one compound, the ratio of the second element that combines with 1 gram of the first element in each is a simple whole number. e.g. H2O & H2O2 water hydrogen peroxide Ratio of oxygen is 1:2 (an exact ratio) The Atomic Theory of Matter • In 1803, Dalton proposed that elements consist of individual particles called atoms. • His atomic theory of matter contains four hypotheses: 1. All matter is composed of tiny particles called atoms. 2. All atoms of an element are identical in mass and fundamental chemical properties. 3. A chemical compound is a substance that always contains the same atoms in the same ratio. 4. In chemical reactions, atoms from one or more compounds or elements redistribute or rearrange in relation to other atoms to form one or more new compounds. Atoms themselves do not undergo a change of identity in chemical reactions. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. The Atomic Theory of Matter • Dalton’s atomic theory is essentially correct, with four minor modifications: 1. Not all atoms of an element must have precisely the same mass. 2. Atoms of one element can be transformed into another through nuclear reactions. 3. The composition of many solid compounds are somewhat variable. 4. Under certain circumstances, some atoms can be divided (split into smaller particles: i.e. nuclear fission). Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. Dalton’s Symbols John Dalton 1808 A Cathode Ray Tube Source of Electrical Potential Stream of negative particles (electrons) Metal Plate Gas-filled glass tube Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58 Metal plate J.J. Thomson • He proved that atoms of any element can be made to emit tiny negative particles. • He knew that atoms did not have a net negative charge and so there must be balancing the negative charge. J.J. Thomson William Thomson (Lord Kelvin) • In 1910 proposed the Plum Pudding model – Negative electrons were embedded into a positively charged spherical cloud. Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56 Spherical cloud of Positive charge Electrons Rutherford’s Gold Foil Experiment Rutherford received the 1908 Nobel Prize in Chemistry for his pioneering work in nuclear chemistry. beam of alpha particles radioactive substance circular ZnS - coated fluorescent screen gold foil Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120 What he expected… What he got… richocheting alpha particles Interpreting the Observed Deflections . . . . . . beam of alpha particles . . . . . undeflected particles . . . . . gold foil Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120 . deflected particle Rutherford Scattering (cont.) Rutherford interpreted this result by suggesting that the a particles interacted with very small and heavy particles Particle bounces off of atom? Case A Case B Particle goes through atom? Particle attracts to atom? Case C Case D . Particle path is altered as it passes through atom? Explanation of Alpha-Scattering Results Alpha particles Nucleus + + - - + + - + + - + - + - - Plum-pudding atom Nuclear atom Thomson’s model Rutherford’s model Bohr’s Model Nucleus Electron Orbit Energy Levels Quantum Mechanical Model Niels Bohr & Albert Einstein Modern atomic theory describes the electronic structure of the atom as the probability of finding electrons within certain regions of space (orbitals). Modern View • The atom is mostly empty space • Two regions – Nucleus • protons and neutrons – Electron cloud • region where you might find an electron mass p = mass n = 1840 x mass e2.2 Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei Mass Number A ZX Atomic Number 1 1H 235 92 2 1H U Element Symbol (D) 238 92 3 1H (T) U 2.3 Do You Understand Isotopes? How many protons, neutrons, and electrons are 14 in 6 C ? 6 protons, 8 (14 - 6) neutrons, 6 electrons How many protons, neutrons, and electrons are 11 in 6 C ? 6 protons, 5 (11 - 6) neutrons, 6 electrons 2.3 Noble Gas Halogen Group Alkali Metal Alkali Earth Metal Period 2.4 A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds H2 H2O NH3 CH4 A diatomic molecule contains only two atoms H2, N2, O2, Br2, HCl, CO A polyatomic molecule contains more than two atoms O3, H2O, NH3, CH4 2.5 ELEMENTS THAT EXIST AS DIATOMIC MOLECULES Remember: BrINClHOF P: 1 or 4 S: 1 or 8 These elements only exist as PAIRS. Note that when they combine to make compounds, they are no longer elements so they are no longer in pairs! An ion is an atom, or group of atoms, that has a net positive or negative charge. cation – ion with a positive charge If a neutral atom loses one or more electrons it becomes a cation. Na 11 protons 11 electrons Na+ 11 protons 10 electrons anion – ion with a negative charge If a neutral atom gains one or more electrons it becomes an anion. Cl 17 protons 17 electrons Cl- 17 protons 18 electrons 2.5 A monatomic ion contains only one atom Na+, Cl-, Ca2+, O2-, Al3+, N3- A polyatomic ion contains more than one atom OH-, CN-, NH4+, NO3- 2.5 Do You Understand Ions? How many protons and electrons are in 27 3+ 13 Al ? 13 protons, 10 (13 – 3) electrons How many protons and electrons are in 78 234 Se ? 34 protons, 36 (34 + 2) electrons 2.5 2.6 A molecular formula shows the exact number of atoms of each element in the smallest unit of a substance An empirical formula shows the simplest whole-number ratio of the atoms in a substance molecular empirical H2O H2O C6H12O6 CH2O O3 O N2H4 NH2 2.6 ionic compounds consist of a combination of cation(s) and an anion(s) • the formula is always the same as the empirical formula • the sum of the charges on the cation(s) and anion(s) in each formula unit must equal zero The ionic compound NaCl 2.6 Formula of Ionic Compounds 2 x +3 = +6 3 x -2 = -6 Al2O3 Al3+ 1 x +2 = +2 Ca2+ 1 x +2 = +2 Na+ O22 x -1 = -2 CaBr2 Br1 x -2 = -2 Na2CO3 CO322.6 2.6 Examples of Older Names of Cations formed from Transition Metals (memorize these!!) From Zumdahl Chemical Nomenclature • Ionic Compounds – often a metal + nonmetal – anion (nonmetal), add “ide” to element name BaCl2 barium chloride K2O potassium oxide Mg(OH)2 magnesium hydroxide KNO3 potassium nitrate 2.7 • Transition metal ionic compounds – indicate charge on metal with Roman numerals FeCl2 2 Cl- -2 so Fe is +2 iron(II) chloride FeCl3 3 Cl- -3 so Fe is +3 iron(III) chloride Cr2S3 3 S-2 -6 so Cr is +3 (6/2) chromium(III) sulfide 2.7 • Molecular compounds • nonmetals or nonmetals + metalloids • common names • H2O, NH3, CH4, C60 • element further left in periodic table is 1st • element closest to bottom of group is 1st • if more than one compound can be formed from the same elements, use prefixes to indicate number of each kind of atom • last element ends in ide 2.7 Molecular Compounds HI hydrogen iodide NF3 nitrogen trifluoride SO2 sulfur dioxide N2Cl4 dinitrogen tetrachloride NO2 nitrogen dioxide N2O dinitrogen monoxide TOXIC! Laughing Gas 2.7 An acid can be defined as a substance that yields hydrogen ions (H+) when dissolved in water. HCl •Pure substance, hydrogen chloride •Dissolved in water (H+ Cl-), hydrochloric acid An oxoacid is an acid that contains hydrogen, oxygen, and another element. HNO3 nitric acid H2CO3 carbonic acid H2SO4 sulfuric acid HNO3 2.7 2.7 2.7 A base can be defined as a substance that yields hydroxide ions (OH-) when dissolved in water. NaOH sodium hydroxide KOH potassium hydroxide Ba(OH)2 barium hydroxide 2.7 2.7 Mixed Practice 1. 2. 3. 4. 5. 6. 7. 8. 9. Dinitrogen monoxide Potassium sulfide Copper (II) nitrate Dichlorine heptoxide Chromium (III) sulfate Ferric sulfite Calcium oxide Barium carbonate Iodine monochloride 1. 2. 3. 4. 5. 6. 7. 8. 9. N2O K2S Cu(NO3)2 Cl2O7 Cr2(SO4)3 Fe2(SO3)3 CaO BaCO3 ICl Mixed Practice 1. 2. 3. 4. 5. 6. 7. 8. 9. BaI2 P4S3 Ca(OH)2 FeCO3 Na2Cr2O7 I2O5 Cu(ClO4)2 CS2 B2Cl4 1. 2. 3. 4. 5. 6. 7. 8. 9. Barium iodide Tetraphosphorus trisulfide Calcium hydroxide Iron (II) carbonate Sodium dichromate Diiodine pentoxide Cupric perchlorate Carbon disulfide Diboron tetrachloride