2 Early ModelsGreek philosophers – 450 B.C. what is the smallest particle?Democritus - Particles are atomosAn atom is the smallest particle of an element that retains the chemical identity of that element.
3 Aristotle Wins Greek society - slave based Beneath them to work with hands – no experimentsSettled disagreements by argumentsAristotle more famous than Democritus Aristotle winsDidn’t believe or look into atoms until…
4 Late 1700s Chemists believed elements: couldn’t be broken down by ordinary meansCombine to form compoundsAdvancements in balances better experiments
5 Laws DiscoveredLaw of Conservation of mass: mass is neither created nor destroyed during chemical or physical changesThen discovered no matter where or how a compound is made it is made of fixed proportions of elements Law of Definite ProportionsEx: Sodium Chloride, table salt, always has 39.34% Na and 60.66% Cl by mass
6 Laws DiscoveredElements combine to form more than one compound Ex: carbon monoxide, CO, and carbon dioxide CO2Law of Multiple Proportions
7 John Dalton 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.
8 Dalton’s Atomic Theory *Atoms cannot be subdivided, created, or destroyedAtoms of different elements combine in whole-# ratios to form compoundsIn a chemical reaction, atoms are combined, separated, or rearranged.
9 1800s – The atom IS divisible Atom has two regions:Nucleus: center of atomProtons: (+) chargedNeutrons: no chargeElectron cloud: region around nucleus containing electrons, (-) chargedSubatomic particles: p+, no, and e-
10 - + J.J. Thomson’s Experiment Cathode - Ray Tube Voltage source Moving electrical current (Cathode Ray) through glass tube with inert gas (Noble Gas)
11 J.J. Thomson’s Experiment Voltage source+-By adding magnets, found the moving (-) particles
12 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.
13 Ernest Rutherford - 1911 Atoms have e- = (-) charge Atoms are neutral need (+) to balanceMust 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
14 Lead block Uranium Gold Foil Florescent Screen Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004
15 His prediction….Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004
16 What he expectedMr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004
17 What he happenedMr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004
18 Results… Most particles passed through 1 in 8000 bounced back Some particles completely reversed path!!!
19 Small dense, positive piece at center, nucleus Plum-pudding is WRONG!Atom is mostly emptySmall dense, positive piece at center, nucleusAlpha particles are deflected by nucleus if they get close.+Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004
20 +Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004
21 Electron Cloud Model Current model of atom Nucleus surrounded by negative cloud “electron cloud”
22 Size of Atom Atom = size of football stadium Nucleus = smaller than a dime in centerElectron = each one smaller than Roosevelt’s eye on the dimeAtom is mostly empty space!
23 Composition of AtomProton has (+) charge equal in magnitude to (-) charge of electronAtoms neutral # p+ = # e-p+ and no about same masse- = 1836 times smaller than p+Nucleus has majority of massNuclear Forces: short-range forces that hold nucleus togetherp+-p+, p+-no, no-no
24 Counting AtomsAtomic Number (Z): number of p+ of each atom of the elementIdentifies Elements!!
25 Atomic Number What is the atomic number for… Hg What is the element with …20 p+Neutral atoms # p+ = # e-In neutral atoms, atomic number = # e-
26 IsotopesDef: atoms of same element with different # of no (different masses)3 H isotopes:Protium: 1 p %Deuterium: 1 p+ 1 no %Tritium: 1 p+ 2 no
27 Mass Number Use mass number to determine isotopes Mass #: # of p+ and n0Atom with 17 p+ and 20 no has mass # of…Chlorine-37 – Hyphen Notation
28 Cl Writing Isotopes 37 17 Nuclear symbol: Mass number Elemental symbol Atomic number
29 Finding # of Neutrons 27Al # of neutrons = mass # - atomic # How many p+, no and e- are in…Selenium – 7927AlOxygen - 1813
30 IsotopesWrite the complete chemical/nuclear symbol for the isotope with 21 protons, 24 neutrons, and 21 electrons.
31 Relative Mass of Atom Mass of Oxygen-16 = 2.656 x 10-23 g Use relative atomic massesPick standard other masses are expressed in relation to standardStandard: Carbon –12 atom
32 Carbon - 12 Has mass of 12 atomic mass units (amu) 1 amu: is exactly 1/12 the mass of a carbon-12 atomCarbon-12: how many p+ and no?1 p+ 1 amu ( amu)1 no 1 amu ( amu)1 e- 0 amu ( amu)
33 Carbon - 12 What element is 4 x mass of Carbon –12? What element is 1/3 mass of Carbon –12?
34 Average Atomic Masses Most elements are mixture of isotopes Mass of element is average mass of isotopesWe need the % of each isotopeAverage Atomic Mass: weighted average of the atomic masses of the naturally occurring isotopes of an element.
35 Weighted Average Box with 100 marbles of 2 sizes: 25% have mass of 2.00g75% have mass of 3.00gWhat is the average mass of a marble?2.75g (sig figs!!)
36 Average Atomic MassOr (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
37 Average Atomic Mass Find the atomic mass of Li if… 7.5% is Lithium – 6 = amu92.5% is Lithium – 7 = amu