lecture #3, atomic structure ( rutherford , bohr models) - mit

Welcome to 3.091Lecture 3

September 14, 2009

Atomic Models: Rutherford & Bohr

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Periodic Table Quiz

Name Grade /10

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Lanthanum

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Courtesy of MIT Student. Used with permission.

The Structure of the Atomstatus report ca. end of the 19th century

* atom is electrically neutral

* -ve charge carried by electrons

* e- has very small mass

bulk of the atom is +ve,

most mass resides in +ve charge

Question: what is the spatial distribution of charge inside an atom?

Figure 1.18


Figure 1.19

Paper0.5- cm lead10-cm lead

γ rays

β particles

α particles


Rutherford-Geiger-Marsden experiment


b

Marsden's Analysis

Scattering of an α-particle which approaches a heavy nucleus with an impact parameter b.

θTrajectory of incident α particle

gold nucleus


principles of modern chemistry:

* recognize patterns

* develop a quantitative model that - explains our observations - makes predictions that can be tested by experiment

Bohr Postulates for the Hydrogen Atom 1. Rutherford atom is correct 2. Classical EM theory not applicable to orbiting e-

3. Newtonian mechanics applicable to orbiting e-

4. Eelectron = Ekinetic + Epotential

5. e- energy quantized through its angular momentum: L = mvr = nh/2π, n = 1, 2, 3,…

6. Planck-Einstein relation applies to e- transitions: ΔE = Ef - Ei = hν = hc/λ

c = νλ






c = νλ

0 500 1000 1500 2000 2500 3000

5000 K

4000 K

3000 K

Rad

iatio

n in

tens

ity

Wavelength (nm)

Classical prediction6000 K

E = hν

Planck suggests that light is composed of energypackets or quanta. The elementary unit of e-mradiation is the photon.

InfraredVisibleUV

6000 K







c = νλ

Isotopes of Hydrogen

hydrogen1766 Henry Cavendish, London

deuterium

1931 Harold Urey, Columbia U. tritium

1934 Ernest Rutherford, Cambridge U.

Dalton proposes theindivisible unit of an element is the atom.

1803

Thomson discovers electrons,believed to reside within a sphere of uniform positive

charge (the "plum pudding" model).

1904

Rutherford demonstrates the existence of a positively chargednucleus that contains nearly all

the mass of an atom.

1911

Bohr proposes fixed circularorbits around the nucleus

for electrons.

1913

In the current model of theatom, electrons occupy regions of space (orbitals) around the

nucleus determined by their energies.

1926


J. J. Thomson (1856-1940)Cathode ray = Charged particle = Electron (1897)charge-to-mass ratio of electron (1897)

J.J Thomson"Plum pudding" model of atom (1904)

Robert Millikan (1868-1953) Charge and mass of electron (1909)

James Chadwick (1891-1974)Neutron (1932)

Ernest RutherfordProton (1920)

Ernest RutherfordNuclear model of atom (1911)

Ernest Rutherford (1871-1937)α and β particles (1898)

Marie and Pierre Curie(1867-1934, 1854-1906)Radioactive elements polonium and radium (1898)

Henri Becquerel (1852-1908)Uranium emits rays that fog photographic film (1869)


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lecture #3, atomic structure ( rutherford , bohr models) - mit

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