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2.1TheNuclearAtom
LearningGoalsUnderstandings:oAtoms containapositivelychargeddensenucleuscomposedofprotons andneutrons (nucleons)
oNegativelychargedelectrons occupythespaceoutsidethenucleus
oThemassspectrometerisusedtodeterminetherelativeatomicmassofanelementfromitsisotopiccomposition
Atom• Thesimplestbuildingblock
• Verysmall
• Anatomisthesmallestunitthatmakesupmatter.
HowSmallisanAtom?
Idea•Democritus (420BCE)firstproposedtheideathatmattermaybemadeupofsmall,indivisibleparticlescalledatoms.
•AtomismdevelopedinChinese&ArabicculturesduringtheDarkAgesinEurope.
Theory
• JohnDalton(1766-1844)wasthefirsttobaseatomictheoryonscientificevidence.
Dalton's Atomic Theory1. All matter is made of tiny indivisible particles
called atoms.
2. No new atoms are created or destroyed.
3. Atoms of the same element are identical, those of different atoms are different.
4. Atoms of different elements combine in whole number ratios to form compounds.
5. Atoms come together to form molecules
Dalton’sModeloftheAtom
• “Billiardball”
Thomson’s Experiment• J. J. Thomson
• Made a piece of equipment calleda cathode ray tube
Voltage source
+_
Metal Disks
• Passing an electric current makes a beam appear to move from the negative to the positive end
Voltage source
+_
Voltage source
• By adding an electric field he found that the moving pieces were negative
+
_
Results
• Found the electron
• “Raison Bun Model”
Rutherford's experiment• Ernest Rutherford
• Worked with alpha particles α
Lead block
Uranium
Gold Foil
Flourescent Screen
What Rutherford Expected…• The alpha particles to pass through without changing direction very much.
• Because…• Positive charges were spread out evenly.
He thought the mass was evenly distributed in the atom
HisExplanation
• Nucleuscomposedofprotonsandneutrons
• Thenucleusis…• Small• Dense• Positivelycharged +
Atom• Atom– smallestparticleofanelement• Atomsaremadeupofsmallerelements,wecallthemsubatomicparticles• Protons• Neutrons• Electrons
• Protonsandneutronstogetherformanucleus
Name Symbol Charge RelativeMass
Electron e- -1 0.0005
Proton p+ +1 1
Neutron n0 0 1
SubatomicParticles
Structure of the Atom• There are 2 regions.
1. Nucleus• Positive charge• Almost all the mass
2. Electron cloud: empty space• The region where the electron can be found.
Nuclear Symbols• Contain the symbol of the element, the mass number and the atomic number.
XA
Z
±
Nuclear Symbols• Contain the symbol of the element, the mass number and the atomic number.
XMassnumber
Atomicnumber
Charge
Massnumberandatomicnumber• Massnumber(A)isthesumofprotons(p)andneutrons(n)
A=p +n
• Thenumberofprotonstellsustheatomicnumber(Z)p= Z
• Charge(±)isp– e
H1
1 H2
1 H3
1Hydrogen Deuterium Tritium
Isotopes• havethesameprotonnumber(Z),butdifferentmassnumbers(A) (sotheyhavedifferingnumbersofneutrons).
• E.g.hydrogenhas3isotopes…
RelativeAtomicMass(Ar)
•Atomsareindividuallytoosmall toweigh.
• Therelativeatomicmassscaleisused,basedontheisotopecarbon-12 becauseitisthemostabundantisotope.
MassSpectroscopy• Theinstrument,massspectrometer,isusedtomeasurethemassofindividualatoms.
• Sincethereareisotopeswitheachelement,yourmassspectrum indicatesthemass/chargeratioofthedifferentionsonthecarbon-12scale.
Howitworks
Agoodvacuumisessentialinthewholeoftheapparatus
HowitworksTheacceleratedionsaredeflected bythemagneticfield,biggerionsaredeflectedless.
Theionbeamscanbefocusedonthedetector
MassSpectrumofZirconium(Zr)Considerasampleof100atoms
50 hasamassof90
10 hasamassof91
15 hasamassof92
20 hasamassof94
5 hasamassof96
𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒𝑎𝑣𝑒𝑟𝑎𝑔𝑒𝑚𝑎𝑠𝑠 = -.-/01/22341567.8/-.12
Classwork• P.64#1– 2• P.66#3– 6• P.68#7– 10