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10-13-16
•  WhatexperimentdidRutherfordperformthat
helpedhimconcludethatthestructureofan
atommustincludeasmall,densenucleus?
Describetheexperimentaldesignand
results...
AtomicComposiBon
•  Protons
–  PosiBve(+)electricalcharge
–  RelaBvemass=1.007atomicmassunits(u)
•  Electrons
–  NegaBve(–)electricalcharge
–  RelaBvemass=0.0005u
•  Neutrons
–  No(0)electricalcharge
–  RelaBvemass=1.009u
So,protonsandneutrons
aboutthesamesize.
Electronsabout1/2000
thesizeofprotonsand
neutrons!
AtomicNumber
Allatomsofthesameelementhavethe
samenumberofprotonsinthenucleus.
Atomicnumber=numberofprotons
13
Al
26.981
Atomic number
Element symbol
Atomic mass*
*Sometimes called Atomic weight
1.Whatistheelementnameandatomic
numberofthefollowingelements:
Ar,Cf,Cs,Se,Cd,Po,Mn,andB
Argon,18
Cadmium,48
Polonium,84
Californium,98
Cesium,55
Selenium,34
Manganese,25
Boron,5
2.Whatistheelementnameandatomic
massofthefollowingelements:
Ca,Ru,Kr,Rb,Cl,U,Ni,andBe
Calcium,40.08u
Chlorine,35.45u
Ruthenium,101.07u Uranium,238.03u
Krypton,83.80u
Rubidium,85.47u
Nickel,58.69u
Beryllium,9.01u
AtomicMass
•  Atomicmasstellsusthemassofoneatomofan
elementrelaBvetooneatomofanother
element.
•  RelaBvemassesoriginallybasedonDalton’s
massof1forHydrogen(H).
•  Forexample,anOatomwithanatomicweight
of15.9994isapproximately16Bmesheavier
thananHatom(atomicweight=1.0079).
MassNumber
•  AtomsofthesameelementALLhavesamenumberof
protons,butcanhaveDIFFERENTamountofneutrons.
•  MassNumberisthesumofprotonsandneutronsin
anatom.
MassNumber=no.ofprotons+no.ofneutrons
•  ABoronatomhasfiveprotonsandfiveneutrons
Massnumber=5p+5n=10
MassNo.
AtomicNo.
10
5B
3.  Whatisthemassnumberofanironatomwith
30neutrons? Atomicno.ofFe=26,so26protons
Massno.=no.ofprotons+no.of
neutrons
=26+30=56
4.  Howmanyprotons,neutrons,andelectronsare
ina64Znatom?
ForZn,massno.=64,atomicno.=30
No.ofneutrons=massnumber–atomicnumber
=64–30=34
30protons,30electrons,34neutrons
5.WhatisthecomposiBonofanatomof
phosphoruswith16neutrons?Whatisitsmass
number?Howwouldyouwritethesymbolfor
thisisotope(includemassnumberandatomic
number)?ComposiWonis15protons,15electrons,16neutrons
Massno.=15+16=31
31
15 P
AtomNumber&MassWorksheet