4 4.1 Early theories and 4.2 Defining the atom 1. Atomists and DemocritusGreeks approx 2,500 years agoMatter was made up of atoms “atomos” or “Indivisible” particlesSeashell experiment—broken into smaller & smaller pieces
5 4.1 Early theories and 4.2 Defining the atom 2. John Dalton; returned to theory of atomsAtoms are like billiard balls (solid spheres) which cannot be broken down further4 major postulates1) All elements are composed of atoms2) Atoms of the same element are identical3) Atoms can physically mix or chemically combine in simple whole number ratios4) Reactions occur when atoms separate, join, or rearrange
6 4.1 Early theories and 4.2 Defining the atom 3. William Crookesdeveloped Crookes tube (CRT) in 1870’sfirst evidence for existence of electrons because you could “see” electrons flow and confirm their existence.tube is precursor to today’s TV picture tubes
7 4.1 Early theories and 4.2 Defining the atom 4. J.J. Thomsondiscovered electron in 1897discovered positively charged particles surrounded by electronsfound the ratio of the charge of an electron to its mass to be 1/1837
8 4.1 Early theories and 4.2 Defining the atom 4. J.J. Thomson
9 4.1 Early theories and 4.2 Defining the atom 4. J.J. Thomsoncathode ray tube experiments – advancement of Crookes tube“plum-pudding model”
10 4.1 Early theories and 4.2 Defining the atom 5. Ernest RutherfordDiscovered nucleus (dense core of atom) in 1911Gold foil experimentsQuote from E.R.’s Lab Notebook“It is about as incredible as if you had fired a15-inch shell at a piece of tissue paper and it came back and hit you.” -ER
11 4.1 Early theories and 4.2 Defining the atom Video Clip: Rutherford Gold Foil Experiment
12 4.1 Early theories and 4.2 Defining the atom 6. Robert MillikenOil drop experimentdetermined the charge and mass of an electronVideo Clip: Milliken Oil Drop Experiment
13 4.1 Early theories and 4.2 Defining the atom 7. James Chadwickdiscovered the neutron (no charge, but same mass as proton)Neutrons help disperse the strong repulsion of positive chargesRelative SizesNucleus diameter = 10-5 nmAtom diameter = 10-1 nmNucleus = basketball --> Atom = 6 miles wide!
14 4.1 Early theories and 4.2 Defining the atom 8. Niels Bohrimproved on Rutherford’s work“planetary model”- positive center is surrounded by electrons in defined orbits circling the center
15 4.1 Early theories and 4.2 Defining the atom defined the following:energy level – the location where an electron is found at a set distance from the nucleus dependent on the amount of energy it hasground state – the typical energy level where an electron is found; lowest energyexcited state – an energy level higher than the ground state for an electron; temporary condition
16 4.1 Early theories and 4.2 Defining the atom 9. Quantum Mechanical ModelErwin Schroedinger; Mathematical modelElectron locations are based on probabilityElectrons are not particles, but waves!Defined:Orbital – region where an electron is likely to be found 90% of the time
17 4.3 How atoms differ Atoms – vocabulary and classifications Atom – the smallest particle of matter that retains its properties.can “see” individual atoms with a scanning tunneling microscope.
18 4.3 How atoms differSubatomic particles – the component parts of an atom: proton, neutron, and electron
19 4.3 How atoms differIon- atom with the same number of protons but a different number of electrons.If the atom has a (+) charge it has fewer electrons than protons, If the atom has a (-) charge it has more electrons than protons.
20 4.3 How atoms differ Subatomic Particle Mass and Abbreviation Charge LocationDiscovererProton1 amu, p++1NucleusNoneNeutron1 amu, nChadwick in 1932ElectronAlmost zero, e--1Electron cloudThomson
21 4.3 How atoms differ Calculations involving Subatomic Particles: atomic number = # of protonsmass number = # of protons + # of neutrons(neutral atom): # of protons = # of electrons(charged ion): charge = #p+ - #e-
22 4.3 How atoms differ Isotopes and Calculations: Isotope – atoms of the same element with different numbers of neutronsAtomic mass – weighted average of the masses of all the isotopes of an element
23 X 4.3 How atoms differ Isotope (Isotopic Notation) Mass # Z Atomic SymbolAAtomic #Example: Uranium-238
24 4.3 How atoms differ Isotope Problems: Multiply the mass number of the isotope by the decimal value of the percent for that isotopeAdd the relative masses of all of the isotopes to get the atomic mass of the element
25 4.3 How atoms differ Example: If 90% of the Beryllium in the world has a mass number of 9 and only 10% has a mass number of 10, what is the atomic mass of Beryllium?
26 4.4 Unstable Nuclei and Radioactive Decay VocabularyRadioactivity-the spontaneous emission of radiation from substancesNuclear reactions- changes in an atom’s nucleusRadiation-rays and/or particles emitted from radioactive material
27 4.4 Unstable Nuclei and Radioactive Decay Types of RadiationAlpha radiation -stream of high energy alpha particlesalpha particles consist of 2 protons and 2 neutrons and are identical to helium-4 nucleus.symbol He 2+2not much penetrating power, travel a few centimeters, stopped by paper, no health hazard
28 4.4 Unstable Nuclei and Radioactive Decay mass number decreases by 4 atomic number by 2alpha decay: Ra 222Ra + 4 HeExample: Uranium-238
29 4.4 Unstable Nuclei and Radioactive Decay Beta radiationhigh speed electronsTo form beta radiation a neutron splits into a proton and an electronThe proton stays in nucleus and the electron propels out at high speed.Symbol 0e- 0e- 0B100 times more penetrating then alpha, pass through clothing to damage skin
31 4.4 Unstable Nuclei and Radioactive Decay Gamma radiationsimilar to X raysdoesn’t consist of particlessymbol: 0penetrates deeply into solid material, body tissue, stopped by Pb or concrete, dangeroususually emitted with alpha and beta radiationno mass or electrical chargeemission of gamma rays by themselves cannot result in the formation of a new atom
32 4.4 Unstable Nuclei and Radioactive Decay Practice:What is the alpha decay of plutonium-250?
33 4.4 Unstable Nuclei and Radioactive Decay 2. What is the beta decay of Carbon-14?