unit 4: atomic theory structure of the atom (& radioactivity)

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Chapter 4:

Unit 4: Atomic TheoryStructure of the Atom (& Radioactivity)Early Atomic Theories

Early Atomic TheoriesAtomists and DemocritusGreeks approx. 2,500 years agoMatter was made up of atoms atomos or Indivisible particlesSeashell experimentbroken into smaller & smaller pieces

Early Atomic TheoriesJohn Dalton1766-1844; returned to theory of atomsAtoms are like billiard balls (solid spheres) which cant be broken down further4 major postulatesAll elements are composed of atomsAtoms of the same element are identicalAtoms can physically mix or chemically combine in simple whole number ratiosReactions occur when atoms separate, join, or rearrange

Daltons Model of the AtomNo subatomic particles!Early Atomic TheoriesWilliam CrookesDeveloped Crookes tube (a.k.a cathode ray tube) in 1870s First evidence for existence of electrons because you could see electrons flow and confirm their existence. Tube is precursor to todays TV picture tubes

Building the Atom The ElectronJ.J. ThomsonDiscovered electron in 1897Discovered positively charged particles surrounded by electrons Found the ratio of the charge of an electron to its mass to be 1/1837Thomsons Cathode Ray Tube Experiment J.J. Thomson

Video: Cathode Ray Tube DemoBuilding the Atom The ElectronJ.J. ThomsonCathode ray tube experiments advancement of Crookes tubeplum-pudding model

Thomsons Model of the Atom (Plum Pudding Model)Millikans Oil Drop ExperimentRobert MillikanOil drop experiment Determined the charge and mass of an electron

Video: Millikan's Oil Drop Experiment

Building the Atom The NucleusErnest RutherfordDiscovered nucleus (dense core of atom) in 1911 Famous Gold foil experiment

Quote from E.R.s Lab NotebookIt is about as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you. -ER

Rutherfords Gold Foil Experiment

Video Clip: Rutherford Gold Foil ExperimentRutherfords Model of the AtomBuilding the Atom The NeutronJames ChadwickDiscovered the neutron (no charge, but same mass as proton)

Neutrons help disperse the strong repulsion of positive chargesNucleus diameter = 10-5 nmAtom diameter = 10-1 nmIf Nucleus = basketball --> then Atom = 6 miles wide!

Building the AtomNiels BohrImproved on Rutherfords workPlanetary model- positive center is surrounded by electrons in defined orbits circling the center

Bohr Model of the Atom (Planetary Model)Bohr Model of the Atom Vocab.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 conditionSchrdinger Model (Quantum Mechanical Model)Quantum Mechanical ModelErwin Schrdinger; Mathematical modelElectron locations are based on probabilityElectrons are not particles, but waves!Interactive Simulation: try it!

Defined:Orbital region where an electron is likely to be found 90% of the timeSchrdinger Model of the Atom(Quantum Mechanical Model)Atomic TheoryAtom the smallest particle of matter that retains its properties.Smallest individual unit of an elementOne atom of hydrogen is different from one atom of carbon.Subatomic particles the component parts of an atom: proton, neutron, and electron

Atomic TheoryIon - atom with the same number of protons but a different number of electrons i.e. an atom with a charge!If the atom has a (+) charge it has more protons than electrons.If the atom has a (-) charge it has more electrons than protons.Subatomic ParticlesSubatomic ParticleMass and AbbreviationChargeLocationDiscovererProtonp+ Mass =1 amu+1Nucleus----NeutronnMass =1 amu0NucleusChadwick in 1932Electrone-Mass zero amu-1Electron cloud (outside nucleus)Thomson in 1897Atomic SymbolsAtomic symbol the letter or letters that represent an element.13Al26.981Atomic numberAtom symbolAtomic mass or weightAtomic NumberAtomic number = the number of protons in the nucleus. (same for every atom of that element)13Al26.981Atomic numberAtom symbolAtomic mass or weightMass NumberMass Number = # protons + # neutrons

A Boron atom can have:5 p + 5 n = 10 amu

Named as boron-10Mass numberAtomic number26Calculations w/ Subatomic ParticlesAtomic number = # of protonsMass number = # of protons + # of neutrons

(For a neutral atom): # of protons = # of electrons(For a charged ion): Charge = #p+ - #e-Isotope NotationIsotope (Isotopic Notation)

Mass #Atomic #Atomic SymbolZAXExample: Uranium-238ExampleExampleSample ProblemWrite the atomic symbols for the following: The isotope of carbon with a mass of 13

The nuclear symbol when A = 92 and the number of neutrons = 146.31IsotopesIsotope atoms of the same element with different numbers of neutrons (different mass numbers)Example: Carbon-12 Carbon-14

Atomic mass weighted average of the masses of all the isotopes of an element

Atomic MassThe weighted average is the addition of the contributions from each isotope.

Isotopic Abundance is the percent or fraction of each isotope found in nature.33Most Abundant Isotope3413Al26.981Atomic numberAtom symbolAtomic mass or weightUsually can round atomic mass on the periodic table to nearest whole number (but not always!!)Example: Determine the average atomic mass of magnesium which has three isotopes with the following masses: 23.98 amu (78.6%), 24.98 amu (10.1%), 25.98 amu (11.3%).

Multiply the mass number of the isotope by the decimal value of the percent for that isotope.Add the relative masses of all of the isotopes to get the atomic mass of the element.35Example: Determine the average atomic mass of magnesium which has three isotopes with the following masses: 23.98 amu (78.6%), 24.98 amu (10.1%), 25.98 amu (11.3%).

36Now You Try!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?Radioactivity - VocabularyRadioactivity - the spontaneous emission of radiation from a substance

Radiation - rays and/or particles emitted from radioactive material

Nuclear reactions - reactions involving changes in an atoms nucleusRadioactivityRadioactive isotopes are unstableThese isotopes decay over time by emitting particles and are transformed into other elementsParticles emitted:Alpha () particles: helium nucleiBeta () particles: High speed electronsGamma () rays: high energy light39Types of Radiation particlesAlpha radiation - stream of high energy alpha particlesAlpha particles consist of 2 protons and 2 neutrons and are identical to helium-4 nucleus.

Symbol: 4He 2+ 2Not much penetrating power, travel a few centimeters, stopped by paper, no health hazard

Types of Radiation particlesBeta radiation - High 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- 0 -1 -1 100 times more penetrating then alpha radiation, pass through clothing to damage skin

Types of Radiation RadiationGamma radiationSimilar to X raysDoesnt consist of particles (instead, high energy light) Symbol: 0 0 Penetrates deeply into solid material, body tissue, stopped by Pb or concrete, dangerousEnd of Unit 4!Models of the Atom



discovery________________100 BCDemocritus/Greeksconcept of the atom


Antoine Lavosier

Law of conservation of mass


Joseph Proust

Law of definite proportions


John Dalton

Law of multiple proportions

Atomic Model I


William Crookes

Cathode Rays (electrons)



Canal Rays (protons)


J.J. Thomson

Plum Pudding Model


Atomic Model II


Ernest Rutherford

nucleus of atom

Atomic Model III


Niels Bohr

Planetary Model

Atomic Model IV


Schroedinger/Planck/ Modern or Wave Model

PresentDeBroglie/Einstein/etc.Atomic Model V


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