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