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Unformatted text preview: 'FnL“OIhhs‘ q," CM kg P .1, ”HI” h cathu
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5w? «crates... wolfJekﬁg CHEM 114A Spring 2004 Mon—Wed Feb 9,111th 2004
Lecture No. 910 website \uwwpublicasuedubcaangell Fe' 9“ Y
Em M 1; 5mm To “Jon.
Chapter 6. Electronic Structure of Atoms. Developing a model for the atom STARTING POINT: Rutherford model of
positively charged nucleus surrounded by negatively
charged cloud of electrons. Key problem: to understand the arrangement and
energy structure of the electrons. wru kw mu 9 Wﬂvﬂrf ht! ‘Nrd'tﬂ. “63.4933? AMBITION to connect the electronic structure to the structure of the periodic table of elements (Rows of 8):
em to.» tunrue; We can a; e HM.“ wmdrv Is Jﬁuﬁ 1 _
tkiﬁ“?uﬁ CLUBS: Special frequencies of radiation emitted from
hot atoms and absorbed by cold atoms LINE SPECTRA what are they? Aditown“: Fm; hm
Imus m Incm... telmm ——3 Nuts.» c. euggg.’ Puumurs In MUCH? UNI: m
TD PREPARE FOR AN UNDERSTANDING OF THIS Em WE NEED TO UNDERSTAND SOMETHING ABOUT
LIGHT ‘53.}! Light is a wave motion in the vacuum of space or in
whatever medium the light is being transmitted through. It is
the electric field which is oscillating in amplitude. It carries
energy which can be absorbed when it ﬁnds a way of exchanging energy with the surr'l‘izﬂgisﬁ 0. Fullh NF 8 ICH 1‘ The velocity of light: c mﬁgutVC'N‘)
3*.g‘MS“ macaw arm? (if F Rnuzmoy . White light: the solar spectrum Fig. 6.4
301' IT? ovum A M! Mr W 7H6 afomomﬂau6?~c «that? JPumuM. NMR 353*. "4". M6 (VH'>UVKI¢M A light wave is characterized by its wavelength
(symbol 2.) or its frequency (symbol v). The two
are related by the velocity of light (symbol c, value 3 X 108 mls.
The speed of light is one of the key constants of nature Equation: 7W = c v C“ u.) Typical question: A light wave is known to have a frequency of 109
cycles per sec. . . What is its wavelength? 0x335==m2mnzimau 3.53.9.3 2 .5033 03352.5 33
macaw Eb e 33 SN» 33 3; 33 S: 3; as Sm Ba 89. 3» 3° 3%: _ _ _ _ _ _ _ Ll 3L5} _
31min 53:5 :< 5 Enqoﬁmg 35 >2. roam. 3&0 Em<ow
ma 0 .2 Eu
_ . a .......... _ _ _ _ _ .  _ ﬂ ._
3; 8; 3; 3; a.“ aim 3; 3» so ,5» 3“ so an is: .1 51o.q . min...N ax Ba «x 8L :3 138:: mqmnm W 00333 Next we need to tell you that light energy comes
in packets called quanta (singular, quantum) Quantum Theory Max Planck, 1900 SOBQYIOH to we u UH‘Gﬁvnwf Cﬁbaﬁhsu cArhcrade"
Luau? £080.“, 64—... sum as ‘3' '"33 N t' {NW(Ivy “Mr GUANVM
Energy of quantum 1s d1rectly proportional to its frequency. We write the equation: I FLA0 “a. gush, E: hv PHo‘re w {I . 1TH; MA a»: (wear
H IS Planck’s constant 1t IS another of E the constants of nature
Value: 6.63 X 10‘34 1.3 Question: A photon of light has a frequency 680 x 106 cyclesfsec. What is its
energy in Joules?
Answer : NEXT STEP: A big insight due to Niels Bohr
(Carlsburg brewery, around 1920). A quantum of light is emitted when an
electron in an atom makes a jump between
distinct energy levels in which it is permitted to exist
Level 1 (high) i It: a hv
NM Level 2 (low) Conversely when a quantum of light energy is absorbed. . ..
In; I T
M ' These processes are both manifestations of the law of conservation
of energy. It can be converted from one form to the other, but not destroyed or created. If Bohr is right we must study the line spectra
of atoms to learn about their energy structure Line Spectra: Balmer Series. Astronomers had observed dark lines in the
solar spectrum, and has sorted them into groups,
one of which, the Balmer Series, was described
by a simple mathematical relation. It was later
found that the same series of lines was emitted from hot hydrogen. This emission spectrum of hydrogen is shown in
Fig 6, 1203) The wavelengths of the lines, numbered all Where n = 3, 4, 5... starting With the lowest
energy line (longest wavelength) which is
found in the red part of the visible
spectrum (see Eq. (6.4) p. 206) 0v IR. 6 Niels Bohr developed a model for the
hydrogen atom based on the following related
ideas: *e move in orbits of “allowed” energy
determined by quantum nos, 1,2,3... * orbits are distinguished from each other in
energy and in distance from the nucleus. 0 the single electron of hydrogen can hop
from one orbit to another, emitting or
absorbing a light quantum depending on
whether it decreases in quantum number,
or increases. The energy Hn of the
quantum depends on the orbits between
which the electron is hopping. The model yielded an exam. prediction of the value of R in the Balmer series equation, and
made predictions of other series of lines not
yet discovered. Yet the model ﬁnally proved to be wrong!! 656.3 _
700 Mum)
Sun Hydrogen Fig. 3.10: Spectrum. (a) the sun and (b)
hydrogen. Energy is on the vertical axis and a
line is drawn for each photon energy observed.
For the sun, there is a line at every energy —
the spectrum is continuous. The hydrogen
spectrum consists of discrete lines. In Bohr’s model, the atom is like a tiny solar
system. The electron is held in place by
electrostatic attraction e' for np”, balanced by
cenw' ;. I ' u on .; ‘. 9' Objects moving in a circular fashion (like a
stone on the end of a string being swung around
a boy’s head) have a Special type of momentum
“angular momentum” = mvr The quantum numbers in Bohr’s model are
determined by the condition (assumption)
that this quantity, the orbital angular
momentum, can only have values differing by integral nos. / p. o... alt'1' (.a M 5.; mvr = n (h/21t) “:M‘LJ‘ “Ham (No explanation for this in the theory
Remember it for later, when de Broglie will explain) When this model is solved a relation between
energy and n is obtained 4"
e
6'
o
1‘
a
s E=const/n2 n=l,2,3.....
(lowest energy for smallest n, E=O for n = inﬁrm: _4h_
11 = 1 is called ground state. Bohr’s theory gave the energy for this IR
ground state in exact agreement “ . ‘
with experiment for the case of (“3) hydrogen atom. Also, the Bohr model gave,
r (radius of orbit of quantum. no. 11) = const/ 112 ...
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 Fall '08
 AKSYONOV
 Chemistry

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