structure of atom
TRANSCRIPT
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John Dalton gave the concept that :
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Chronology
Cathode Rays & Discovery of Electrons
Alpha, Beta, Gamma Rays
Thomson's Atomic Model
Rutherford's Atomic Model
Bohr model – electron arrangement
Neutron discovered – Chadwick (1932)
Isotopes & radioactivity
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The line diagram of electric discharge tube is given :
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Atoms must have at least two types of particles
Some should have +ve charge, some –ve
Number of particles having +ve charge should be equal to number of particles of –vecharge
Atom is electrically neutral
+ve charge ---- protons
-ve charge ---- electronsNeutrons
are still not discovered
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Atom is like a spherical ball
Positive electric charge is uniformly spread on
its total volume
Protons possessing +ve electric charge and
Electrons possessing –ve electric charge are
arranged at definite places
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If the atom is like a sphere (Thomson model)
then all Alpha rays should return back after
striking foil.
Rutherford observed that most rays passed
straight through and only very few got deviated
or got thrown back
Proportion of Alpha rays returning and those
passing through was 1 : 1200
So, Mr. Thomson were wrong!!!
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Atom is hollow
Centre – heavy, very small, positively charged nucleus
Electrons – negligible mass, negatively charged are arranged around nucleus
to summarize…..
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i) 1st orbit (K orbit) – 2 electrons
ii) 2nd orbit (L orbit) – 8 electrons
iii) 3rd orbit (M orbit) – 18 electrons
iv) 4th orbit (N orbit) – 32 electrons
2nd, 3rd and 4th orbit have sub-orbits
called Orbitals
First electrons fill 1st orbit then go
stepwise to 2nd, 3rd and 4th orbits
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Do not possess any electric charge
It is neutral
Mass almost equal to mass of
proton
1838 times more mass than
electron
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If the number of protons remain same (atomic
number) but number of neutrons vary than the
element will have different masses
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O16 , O17, O18
Isotopes of Uranium
U235
92U232
92 U238
92
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* The isotopes of elements having high atomic masses
possess the property of radioactivity
* Not all high mass elements are radioactive
* Eg. Lead is not radioactive
* As atomic number increases, number of protons also
increase
* If no. of neutrons also increase the atom becomes
unstable
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Determining age of old trees
Age of fossils
Radiometric dating
Medical treatments
Industries
Treatment of diseases like
cancer, etc.
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