3 THE FIRST ATOMIC SCIENTIST DEMOCRITUS (400 BC)First scientist to hypothesize that all matter was made of small particlesNamed the small particles atomsAtoms in Greek means indivisibleDemocritus thought these particles could not be broken down into smaller units, indivisible and indestructibleOnly theory, did not have “hard” evidenceHis ideas were challenged by Aristotle and PlatoAristotle (360 BC) argued that atoms did not exist
4 FOUNDATIONS LAW OF CONVERSATION OF MASS LAW OF DEFINITE PROPORTIONS Antoinne Lavoisier clearly formulated it in 1789Mass is neither created nor destroyedLAW OF DEFINITE PROPORTIONSJoseph ProustSpecific substances always combine in the same ratio; i.e. a chemical compound always contains the same proportion of elements by mass. (Water is always 2 hydrogen and 1 oxygen)Experiments conducted between 1797 and 1804What John Dalton used beginning in 1803 to formulate the basis of his atomic theory
5 FIRST ATOMIC THEORY JOHN DALTON (1808) English scientist who investigated Democritus claim of atomsDalton created the first “accepted” Atomic Theory by using experimental methodsAll matter is made up of small particles called atomsAtoms of a given element are identical in size and massAtoms of different elements can physically mix together or chemically combine to form compoundsIn reactions, atoms combine, separate, or rearrangeAtoms cannot be divided, created, or destroyed
6 MODERN ATOMIC THEORY 2 KEY CHANGES TO DALTON’S THEORY Atoms are divisible into smaller particlesGiven elements can have atoms of different masses
8 PARTICLES IN THE ATOM DISCOVERY OF THE ELECTRON J.J. Thomson (1897) Created an experiment to test sending electric currents through gases at low pressureCathode Ray ExperimentOne end, anode, became positively charged.Other end, cathode, became negatively charged.Once charged, a glowing beam appeared that traveled from the cathode to the anode (cathode ray).Since the atoms are suppose to be neutral, then why would they atoms be attracted to the positive plate?Hypothesized that there was some negative particle in the atom that was attracted to the positive plateNamed the negative particle a corpuscle
9 PARTICLES IN THE ATOM DISCOVERY OF THE ELECTRON J.J. Thomson With the discovery of the electron, developed the Plum Pudding ModelPictured plum like negatively charged electrons embedded in a sphere of positively charged pudding like “goo”
10 PARTICLES IN THE ATOM CHARGE AND MASS OF THE ELECTRON Robert Millikan conducting the infamous Oil Drop Experiment in 1909 (and again in 1913 due to infighting with another physicist) was able to calculate the specific mass of a single electron and prove that it does in fact have a negative electric chargeMass of an electron is x kg
11 PARTICLES IN THE ATOM DISCOVERY OF THE PROTON Eugen Goldstein (1886) Observed a cathode-ray tube and found that rays could also travel in a tube toward the cathode (negative end)Since opposite attracts, there had to be a positive particle in the atom, called these things canal raysPositive particles are protons and have a mass about 1840 times larger than an electron(Sometimes because his colleagues did not agree with him, he is often not given credit with discovering the proton)
12 PARTICLES IN THE ATOM DISCOVERY OF THE NEUTRON James Chadwick (1932) Found high energy particles with no charge and roughly the same mass as a proton (neutron is slightly larger)Neutron has no charge
13 THE CENTER OF THE ATOM THE NUCLEUS Ernest Rutherford (1911) Gold Foil ExperimentA piece of gold foil was bombarded with alpha particles (positively charged)Assumed the positively charged and electrons were evenly distributed throughout the gold (Plum Pudding Model)Of 8,000 alpha particles fired, 7,999 “stuck” to the foil (because opposite attract and the + alpha particles were attracted to the – electrons in the Au atoms1 in 8,000 particles bounced back. These alpha particles must have found the protons in the Au atoms
14 THE CENTER OF THE ATOM THE NUCLEUS Ernest Rutherford Gold Foil Experiment showed only a small amount of space occupied by positive particlesConcluded that almost all the mass and positive charge of an atom is located in the center (the nucleus) and the electrons are just scattered around and orbit the nucleus like the planets orbit the sunLater found that nucleus contains protons (positive particles) and neutrons (particles with no charge)
15 FORCES IN THE NUCLEUSNucleus held together by short range nuclear forcesShort range forces include:Proton-ProtonProton-NeutronNeutron-Neutron
16 ACCEPTED THEORY TODAY Based on Wave Mechanics Center of the atom is called the nucleusContains the protons and the neutronsElectrons “float” in the electron cloudElectron cloud broken up into energy levelsElectrons’ distance from the nucleus depends on their energyAll matter is made up of atomsAtoms from different elements combine to form compounds in chemical reactions; can also physically combineAtoms rearrange, separate, combine in chemical reactions
17 IDENTIFYING ATOMSAtomic NumberNumber of protons in the nucleus of an atom of that elementPeriodic table is arranged according to increasing atomic numberThe atomic number identifies the elementBecause atoms are electrically neutrally, the number of protons equals the numbers of electrons
18 IDENTIFYING ATOMS Mass NumberTotal number of protons and neutrons IS NOT ON THE PERIODIC TABLE!!!!!!!!Atoms of the same element do NOT have to have the same number of neutrons (isotopes)Isotope: atoms of the same element that have different masses due to different numbers of neutronsNumber of neutrons = mass # - atomic #Example: Carbon with a mass number of 15
19 IDENTIFYING ATOMS Average Atomic Mass Since not all atoms of the same element have the same mass (because of the existence of isotopes), the mass on the periodic table (the decimal number) is the average atomic massThis mass is given in atomic mass units (amu)To find the average atomic mass: identify all known isotopes of an element and record their masses and determine the average
20 IDENTIFYING ATOMS Average Atomic Mass Examples: Hydrogen Formula: (%abudance/100)(mass)+(%abundance/100)(mass)Neon-20 has a mass of amu and Neo-22 has a mass of amu. In an average sample of 100 Neon atoms, 90 will be Neon-20 and 10 will be Neon-22. Calculate the average atomic mass.
22 MOLE CONVERSIONS WITH COMPOUNDS MOLE REVIEWOne mole of ANY element is equal to 6.02x1023 atoms of that elementThe mass (in grams) of one mole of any element is found on the periodic table underneath the symbolExample: 45 grams of Na = ? MolesExample: 7.8x1045 atoms of K = ? g of K
23 MOLE CONVERSIONS WITH COMPOUNDS 56 grams of Al2O3 = ? Moles of Al2O3Molar Mass of a CompoundDetermine how many of each element is present in the compoundMultiply the mass of the element times the number of that element presentAdd the masses togetherConvert using the fence method45 moles of H2O = ? of H2O9.4x1035 atoms of AmO = ? g of AmO
24 NUCLEAR RADIATION DISCOVERY Henri Becquerel Marie Curie Uranium salts and photographic platesConducted experiment twice, once on a sunny day and once on a cloudy dayMarie CurieNamed the process by which particles give off rays radioactivitySpent entire scientific career working on advancements within nuclear radiation fieldWon two Nobel Prizes for work; one was shared with husband and Becquerel, one was independently wond
25 RADIATION DECAY RADIOACTIVE DECAY NUCLEAR STABILITY When a nucleus spontaneously disintegrates into a lighter, more stable elementWhen this happens, x rays and radiation are given offNUCLEAR STABILITYIn nuclear reactions, the nuclei of unstable isotopes (radioisotopes) gain stability by undergoing changes. These changes will continue until stability is reached.
26 RADIATION DECAY 6 TYPES OF RADIOACTIVE DECAY Alpha DecayBeta DecayPositron DecayGamma DecayProtonNeutronThe neutron-to-proton ration determines the type of decay that occurs
27 RADIATION DECAY TYPE SYMBOL Alpha (helium nucleus) a, 4 He 2 Beta (Electron)b, 0e-1Positron (particle with the mass of an electron but with a positive charge)+1Gamma (high energy, electromagnetic)gProton1pNeutronn
28 TYPES OF RADIATION (NUCLEAR REACTIONS) 3 MAIN TYPESAlpha RadiationConsists of a helium nuclei (alpha particle) emitted from a radioactive source; alpha particle emitted which contains two protons, two neutrons and has a double positive chargeBeta RadiationAn electron resulting from the breaking apart of a neutron in an atom; the neutron breaks apart into a proton. All that remains is a nucleus; the electron is released (beta particle)Gamma RadiationHigh-energy photon; electromagnetic. Nuclei often emit gamma rays along with alpha or beta particles during radioactive decay; (gamma ray-particle) no mass or electrical charge
29 NUCLEAR REACTIONS EXAMPLES 32 __ 28 P __ + Al 15 __ 13 14 ___ 14 __P __ Al_____C ___ N______Ra n ______
30 NUCLEAR REACTIONS TRANSMUTATION The conversion of an atom of one element to an atom of another element (via changing the number of protons in the element)Two ways it occursRadioactive decayParticles bombard the nucleusAllows chemists to produce elements that do not occur naturallyThese elements have atomic numbers greater than 92Earliest transmutation took place in 1919 by Rutherford; took Nitrogen-14 and formed an unstable isotope of Fluorine (Fluorine-18); it was this experiment that led to the discovery of the proton
31 NUCLEAR STABILITY NUCLEAR STABILITY Helium has 2 protons, 2 neutrons, 2 electronsAtoms only made of protons, neutrons, electronsTo find the mass of one atom Helium, add up masses of the 3 particlesProton = amuNeutron = amuElectron = amu2(1.007) + 2(1.009) + 2(0.001) = amuIf you put that same atom on the scale, the mass is only amuMass Defect
32 NUCLEAR STABILITY MASS DEFECT The EINSTEIN CONNECTION Difference between adding up the masses of all the particles in an atom and the mass of the actual atomThe EINSTEIN CONNECTIONE = mc2Mass moving at high speed can be converted to energyMass defect comes because some of the mass is changed into energyNUCLEAR BINING ENERGYEnergy released when nucleus is formedThe higher the energy, the more stable the atom
33 NUCLEAR FISSION Splitting of a nucleus into smaller fragments Uranium-235 and plutonium-239Started with neutron bombardmentContinued via chain reactionNeutrons produced react with other fissionable atoms, producing more neutrons which react with still more fissionable atomsContinues until nucleus stability is reachedControl fission in a nuclear reaction by neutron moderation and nuclear absorptionNuclear Power PlantsHigh nuclear wasteAtomic bomb
37 NUCLEAR REACTIONS HALF LIFE The time required for one-half of the nuclei of a radioisotope sample to decay to productsAfter each half-life, half of the existing radioactive atoms have decayed into atoms of a new element
38 NUCLEAR REACTIONS HALF LIFE Example 1: If Polonium-32 has a half life of 14.3 days and you start with 4.0 mg, how many mg with you have after 57.2 days?Step 1: How many half lives with the atom go through?(time you wait)/(half life length) = 57.2/14.3 = 4.00Step 2:Amount left = starting amount * (1/2)#half livesAmount left = (4.0)*(1/2)4.00 = ?????
39 NUCLEAR REACTIONS HALF LIFE Example 2: The half life of radon-222 is days. If you wait for 15 days and find 50 grams of radon-222, how much did you start with?