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Atomic Structure The Idea of the Atom Early Models Greek philosophers – 450 B.C. what is the smallest particle? Democritus - Particles are atomos An atom is the smallest particle of an element that retains the chemical identity of that element. Aristotle Wins Greek society - slave based Beneath them to work with hands – no experiments Settled disagreements by arguments Aristotle more famous than Democritus Aristotle wins Didn’t believe or look into atoms until… Late 1700s Chemists believed elements: couldn’t be broken down by ordinary means Combine to form compounds Advancements in balances better experiments Laws Discovered Law of Conservation of mass: mass is neither created nor destroyed during chemical or physical changes Then discovered no matter where or how a compound is made it is made of fixed proportions of elements Law of Definite Proportions Ex: Sodium Chloride, table salt, always has 39.34% Na and 60.66% Cl by mass Laws Discovered Elements combine to form more than one compound Ex: carbon monoxide, CO, and carbon dioxide CO2 Law of Multiple Proportions John Dalton 1. 2. 1808 –DALTON’S ATOMIC THEORY All matter is composed of small particles called atoms. *All atoms of a given element are identical, but they differ from those of any other element. Dalton’s Atomic Theory 3. 4. 5. *Atoms cannot be subdivided, created, or destroyed Atoms of different elements combine in whole-# ratios to form compounds In a chemical reaction, atoms are combined, separated, or rearranged. 1800s – The atom IS divisible Atom has two regions: Nucleus: center of atom Protons: (+) charged Neutrons: no charge Electron cloud: region around nucleus containing electrons, (-) charged Subatomic particles: p+, no, and e- J.J. Thomson’s Experiment Cathode - Ray Tube Voltage source - + Moving electrical current (Cathode Ray) through glass tube with inert gas (Noble Gas) J.J. Thomson’s Experiment Voltage source + • By adding magnets, found the moving (-) particles Thomson’s Model Same properties no matter what element used All atoms had ELECTRONS!! Thomson’s Model Plum pudding: bunch of positive stuff, with the electrons able to be removed. Ernest Rutherford - 1911 Atoms have e- = (-) charge Atoms are neutral need (+) to balance Must be other particles to account for mass of atom… Rutherford used alpha particles to test. Alpha particle: (+) charge, (2 p+ and 2 no) Experiment: beam of high speed alpha particles into thin gold foil Lead block Uranium Florescent Screen Gold Foil Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004 His prediction…. Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004 What he expected Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004 What he happened Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004 Results… Most particles passed through 1 in 8000 bounced back Some particles completely reversed path!!! Plum-pudding is WRONG! Atom is mostly empty Small dense, positive piece at center, nucleus Alpha particles are deflected by nucleus if they get close. Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004 + + Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004 Electron Cloud Model Current model of atom Nucleus surrounded by negative cloud “electron cloud” Size of Atom Atom = size of football stadium Nucleus = smaller than a dime in center Electron = each one smaller than Roosevelt’s eye on the dime Atom is mostly empty space! Composition of Atom Proton has (+) charge equal in magnitude to (-) charge of electron Atoms neutral # p+ = # ep+ and no about same mass e- = 1836 times smaller than p+ Nucleus has majority of mass Nuclear Forces: short-range forces that hold nucleus together p+-p+, p+-no, no-no Counting Atoms Atomic Number (Z): number of p+ of each atom of the element Identifies Elements!! Atomic Number What is the atomic number for… Hg What is the element with … 20 p+ Neutral atoms # p+ = # eIn neutral atoms, atomic number = # e- Isotopes Def: atoms of same element with different # of no (different masses) 3 H isotopes: Protium: 1 p+ 99.9885% Deuterium: 1 p+ 1 no 0.0115% Tritium: 1 p+ 2 no Mass Number Use mass number to determine isotopes Mass #: # of p+ and n0 Atom with 17 p+ and 20 no has mass # of… Chlorine-37 – Hyphen Notation Writing Isotopes Nuclear symbol: Mass number Atomic number 37 Cl 17 Elemental symbol Finding # of Neutrons # of neutrons = mass # - atomic # How many p+, no and e- are in… Selenium – 79 27Al 13 Oxygen - 18 Isotopes Write the complete chemical/nuclear symbol for the isotope with 21 protons, 24 neutrons, and 21 electrons. Relative Mass of Atom Mass of Oxygen-16 = 2.656 x 10-23 g Use relative atomic masses Pick standard other masses are expressed in relation to standard Standard: Carbon –12 atom Carbon - 12 Has mass of 12 atomic mass units (amu) 1 amu: is exactly 1/12 the mass of a carbon-12 atom Carbon-12: how many p+ and no? 1 p+ 1 amu (1.007276 amu) 1 no 1 amu (1.008665 amu) 1 e- 0 amu (0.005486 amu) Carbon - 12 What element is 4 x mass of Carbon –12? What element is 1/3 mass of Carbon –12? Average Atomic Masses Most elements are mixture of isotopes Mass of element is average mass of isotopes We need the % of each isotope Average Atomic Mass: weighted average of the atomic masses of the naturally occurring isotopes of an element. Weighted Average Box with 100 marbles of 2 sizes: 25% have mass of 2.00g 75% have mass of 3.00g What is the average mass of a marble? 2.75g (sig figs!!) Average Atomic Mass Or (atomic weight) found on periodic table (ROUND TO 2 DECIMAL PLACES!!) Average Atomic Mass = (% Iso. #1)(Mass Iso.#1) + (%Iso.#2)(Mass Iso.#2) + … 100 Average Atomic Mass Find the atomic mass of Li if… 7.5% is Lithium – 6 = 6.015 amu 92.5% is Lithium – 7 = 7.016 amu Example – AAM = 16.00 amu Isotope Fractional Abundance Oxygen – 16 (15.99 amu) 99.757% Oxygen – 17 (17.00 amu) .038% Oxygen – x .235% The Mole Quantities 1 dozen = 1 gross = 144 1 ream of paper = 12 500 In chemistry: 1 mole = 6.022 x 1023 The Mole SI unit for amount of substance (mol) Def: the number of particles in exactly 12 g of carbon – 12. 12 g of carbon –12 has 6.022 x 1023 atoms Avogadro’s #, after Amadeo Avogadro The number is HUGE!! Molar Mass Def: mass of 1 mole of a pure substance 1 mole Carbon –12 = 12 g 1 atom Carbon – 12 = 12 amu Mass of 1 mole of He atoms? 4.00g/mol Same as atomic mass from periodic table, different units Molar Mass So: 4.00g He, 6.94 g Li and 200.59 g Hg all have 6.022 x 1023 atoms = 1 mole!!! Molar mass (g/mol) conversion factor!! Gram/Mole Conversions Mass, in g, of 3.6 mol of C? How many moles are in 23.5 g S? 43g 0.733 mol p. 85 problems Conversions with Avogadro’s # Diatomic Molecules: elements that exist as 2-atom molecules in natural state 7 diatomic molecules H2, O2, N2, F2, Cl2, Br2, I2 Remember!!! HONClBrIF Conversions with Avogadro’s # How many moles of oxygen are in 2.00x1022 molecules of oxygen? How many atoms of sodium are in 3.80 mol of sodium? Conversions with Avogadro’s # How many molecules of hydrogen are in 0.020 g of hydrogen? Mass of 5.0x109 atoms of neon? p.86 problems