the story of atom

7/28/2019 The Story of Atom

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Daltons Model (1803)


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He was the Professor of Physics at and director of Cavandish(physicist who discovered Hydrogen) Laboratory at

Cambridge University


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At high voltage (10-20 kV) and low pressure (0.0001 mm of Hg)


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There is no concept of Protons

The electrons are mobile


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Professor of Physics at Victoria University in Manchestor , UK Came from Newzelend under Scholarship to Cambridge Universityanddid his Phd. under J.J Thomson

Characterized alpha and beta particles Got Noble prize for his work


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Marsden did the calculation for Rutherford and calculated that

R(nucleus)/R(atom) = 1/10000Marsden was a school dropout


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Rutherfords Model of Atom (1911)


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Strongve reaction to Rutherfords Model

Q 1. Why doest Nuclear collapse?

Q 2. Why doest the electrol runs out of energy?

Where the energy is coming from?


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Neil Bohr

Completed his Phd. fromCopenhegan in 1912.

Got scholarship for research

Spends 6 months with J.JThompson and 3 monthswith Rutherford and then hepurposed his atomic model


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B h P l f h (H d ) A


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Bohr Postulates for the (Hydrogen) Atom(1913)

Rutherford atom is correct.

Classical EM theory not applicable to orbitinge-

Newtonian mechanics applicable to orbiting e- Eelectron = Ekinetic + Epotential

electron energy quantized through its angular

momentum i.e. L = mvr = nh/2, n = 1, 2,3,

Planck-Einstein relation applies to e-

transitions i.e. E = EfEi = h = hc/


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Bohrs Model of atom: Planetary model

Eelectron = Ekinetic + Epotential= 1/2mv + k.(q1. q2)/r

= 1/2mv - k.Ze/r.(1)

Fnet = 0, mv/r + k.q1.q2/r

mv/r - Ze/r...(2)

mvr = n.h/2 where n=1,2,3,. .(3)


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On solving these equations

1. R(n) = K1n/Z

2. E(n) = -K2Z/n

3. V(n) = K3Z/nFor Hydrogen atomZ=1, n=1 (ground state)

R(1)= 0.529 AE(1)= -13.6 eVV(1) = 2.18 x 10^6 m/s which is 1% of speed of

light.


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Prism Spectrograph A.A. ngstrm(1853)


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J.J Balmer (1885) He Studied Angstroms data and found the pattern as:

Wave number (1/) = R.[ - 1/n] n= 3, 4, 5, 6where R is constant = 1.1 x 10^7 /m


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Q. Why didt Angstrom find the emission from n=2 to n=1?

Ans: (2-1) = 122nm


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James Frank and Hertz (1913), Berlin They work on Gas discharge tube containing Hg

vapours

For more information on this experiment go tohttp://hyperphysics.phyastr.gsu.edu/hbase/frhz.html
http://hyperphysics.phyastr.gsu.edu/hbase/frhz.htmlhttp://hyperphysics.phyastr.gsu.edu/hbase/frhz.html


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I.E for Hg = 10.4 eV

Q: What does it mean?It means the energy of everyelectron in any atom is quantized!!

For more information on this experiment go tohttp://hyperphysics.phyastr.gsu.edu/hbase/frhz.html
http://hyperphysics.phyastr.gsu.edu/hbase/frhz.htmlhttp://hyperphysics.phyastr.gsu.edu/hbase/frhz.html


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Limitations of Bohrs Model

In 1887 Michalson and Morley in Clevelandthey were looking at the angstroms data andnoticed that the line from 3 to 2 transition is

thicker.

Why is it so?

Bohrs model was silent about it


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In 1896 holand, Zeeman was doin his Postdoc. UnderLorentz in magnetic field.

They put the gas discharge tube under apowerful magnet

This is known as line splittingSplitting is directly proportional to BBoth Lorentz and Zeeman got noble prize forTheir work


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In 1913, Germany, Stark put gas discharge

tube in electrical field He found the same line splitting.

Splitting is directly proportional to E

Bohrs Model was silent on line splitting


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Modifications to Bohrs Model

In 1916, Sommerfield purposed Keplersplanatery model in atom

Sommerfield on his 80th birthday published


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Sommerfield on his 80th birthday publisheda book on his research work

Caption: Circle is the degenerate form of ellipse

St G l h i t (1921)


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Stern Garlach experiment (1921)

Stern won the noble prize for his research work

For more information on this experiment go tohttp://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment
http://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experimenthttp://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment


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Goudsmit and uhlenbeck (1925) They purposed the idea of electron spin and

give spin quantam no. (s= +1/2, -1/2)

Q: Why do they choose Silver?


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Quantam numbers

Principle quantam no (n) Neils Bohr (1913)

Orbital quantam no (l) Sommerfield (1916)

Orientation quantam no (m) Sommerfield (1916)

Spin quantam no (s) Goudsmit and uhlenbeck (1925)


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Pickering 1896

He was studying the spectrum of a star light in HarvardUniversity.

He said that he found atomic hydrogen spectrum.

In 1912 Fouller in London confirmed the same spectrum.

Bohr look at the data and said that its not atomichydrogen but He+ so your lines are off by a factor of 4.

Fouller calculated and said that Mr. bohr the lines are

actually off by a factor of 4.0016 Bohr took mass of nucleus under consideration and

reduced mass= [1/(1/m1)+1/(1/m2)] and calculated thatit should be off by a factor of 4.00163


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Modified energy level diagrams

Aufbou Principle: the word Aufbou meansconstruction. It tells us about the filling sequenceabout of electrons in different orbitals

Pauli exclusion principle: Pauli was an austrian, didhis Phd. under sommerfield in munikh and his Postdoc. Under Neils Bohr and He won noble prize forhis research work.

1. It states that any electron a uniqe set of fourquantam numbers (n, l, m, s)

2. Electrons fills in orbitals from lowest to highestenergy. Energy is a function of four set of quantamnos.

3. Hunds Rule: Orbitals for equivalent energy will

strive for unpaired electron (spins)


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Wave mechanics

Lous victor de Broglie (1924): was doinggraduation in political science, left the collegeand did Phd. in Physics.

He asked (in his thesis) if a photon which has

no mass can behave as a particle does thatfollow that an electron which has mass canbehave as a wave?

He proved that electron behaves as a wave andthat = h/p = h/me.v


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Davission and Germer (1927)

They were studying metal crystals by X-Rayanalysis.

Diffraction is explained only by consideringwave property


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Wave particle duality is complete

Matter behaving as a wave so it was named aswave mechanics


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Warner Hiesenberg: did his phd. in Munikhand then his post doc. Under Neils Bohr.

Got fad up with his work, took a leave and to aiseland in Norway.

3 weeks later he came up with themathematical formulation of quantammechanics

Heisenbergs Uncertainty principle: Theposition and velocity of an electron can not bedetermined accurately after a certain limit.

p.x>=h/2

E i S h di


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Ervin Schrodinger


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What do we learn?We learn about the Electronic

structure in an atom and that itdictates the properties of

atoms/elements

the story of atom

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