History of the atom: History of the atom: Changing atomic models.
Models of the Atom
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Models of the Atom
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Models of the Atom
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1907 Plum Pudding Model - Thomson
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Rutherford Model 1911
Ernest Rutherford “atoms contain a very small heavy central positive nucleus, with the e-orbiting randomly around.
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Alpha particles are He nuclei 2p+, and 2no.
2 elementary charges.
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Most particles went straight through, but the ones that passed closest the Au nucleus were progressively more deflected.
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Gold foil experiment :atom is mostly empty space
with dense positively charged nucleus.
Neg e- move in circular orbits about the +nucleus.
e- attracted to nucleusby electrostatic F
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-inertia from circular velocity of e- (angular momentum) balanced the electrostatic attraction of the nucleus.
What kept the neg e- from fall into the nucleus?
+
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Problems:
• James Maxwell had proved earlier that accelerated charges radiate EM energy.
• Since e- is in circular motion it is accelerated.
• e- should lose E & spiral into the nucleus.
• That does not happen!
• Also - How did positive nucleus stay together?
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One interesting discovery of Rutherford’s experiment was he could estimate the diameter of the nucleus.
He was able to use the repulsion of the alpha particle & the angle of deviation to estimate the diameter of the gold nucleus.
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Angle of Deflection
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The particle repelled straight back would have to come to rest for a moment. At that moment its KE would be balanced by electrical PE.
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Angle of deviation from undeflected path. Rutherford used scattering angles from many
particles to make his measurement.
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KE = E elc.
KE = kQq/r
• Q = charge on nucleus
• q = charge on alpha particle
• r is the “distance of closest approach”
see table p 8 topic 9 V = kq/r.
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Ex 1: An particle with KE = 7.7 MeV aimed at a gold nucleus is repelled straight back. Find the
distance of closest approach.
• 3 x 10-14 m.
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• IB Questions Rutherford.
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Bohr proposed working model for H.
• e- circles nucleus.
• Fc provided by Felc keeps e- in orbit.
• Only orbits with certain radii allowed.
• Larger radius orbits require more e- energy for e- to occupy.
• Electrons jump between orbits somehow without occupying space between.
• Take “Quantum Leap”
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• Ground state = lowest possible e- energy.
• Electrons emit photons of E, when falling to ground.
• Electrons absorb photons of E, when jumping to higher/larger radii orbits.
• Since E conserved, E emitted as photon of EM as e- falls.
• E = Ef – Ei = hf.
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Evidence for Bohrcomes from emission and absorption spectra of light.
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Electric E supplied to gas tubes causes gases to emit light.
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Emission SpectrumWhen viewed through a prism or
spectroscope, we see only certain of light are emitted by each element.
Bright Line Spectra
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Continuous spectrumFrom sunlight
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Frequencies emitted exactly match the frequencies absorbed.
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Quantization
• Since e- can only occupy certain orbits, the orbits themselves are quantized!
• To “jump” to a higher orbit, an e- absorbs an exact amount of energy equivalent to the difference between the E of the two orbits.
• If the E is more than the difference, no jump will occur.
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• Light is produced during e- transitions.
• It is not continuous but quantized in packets – photons.
• A beam of light is made of trillions of photons produced from e- transitions.
• More photons = brighter light.
• Think of higher f photon as more massive – higher momentum.
Summary
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Diagrams
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Orbital Energy Levels/ Ionization Energy
Each orbit is associated with a specific energy which corresponds to the minimum energy needed to totally strip an e- from that orbit.
This ionization energy is more than the energy needed to jump between orbits.
If an atom absorbs E equal to the orbit E it becomes ionized (charged).
Orbits are named by quantum number/letter.
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Ex 2: How much energy would be needed to ionize an electron:
In the n=1 level of of Hydrogen?
in the n = b or level of Mercury?
In the n = 2 level of Hydrogen?
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Atoms must also absorb energy for the e- to jump to higher orbits.
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The amount of energy needed to jump up must exactly equal the E difference btw orbits.
Ephoton = Ei - Ef
Use Ephoton = hf of the radiation.
to find frequency associated with photon of known energy.
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Ex 3: a) How much E is absorbed when a H e- jumps from n=1 to n=3?
B) If the e- drops back down to the n=1 orbit, what f photon is emitted?
C) To which type of radiation does that photon correspond?
D) How many different photons are possible to be emitted by electron dropping from the n=3 to n=1 level?
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n =3 to n = 1 Ephoton = Einitial - Efinal.
-13.6 eV - (-1.51 eV)= -12.1 eV
(12.1 eV)(1.6 x 10-19 J/eV) = 1.936 x 10-18J.
E = hf. f = E/h
f = 1.936 x 10-18J/(6.63 x 10-34 Js)
f = 2.92 x 1015 Hz. Look up.
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Ex 4: A Mercury Atom has an e- excited from the n=a to the n=e energy level.
• What is the frequency it will absorb?
• To which radiation does the frequency correspond?
• If the e- drops down from the e to the b level, what type of radiation will it emit.
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Homework Set
• Read Hamper 7.1 pay attention to purple box. Do 1 – 4 page 149 and
• IB packet Bohr Model prb
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Hist of Quantum pt 1 British 15 min Max Planck and E= hf.
• http://www.youtube.com/watch?v=zBTbqOgdfEY
Bohr Model 6 min
• http://www.youtube.com/watch?v=-YYBCNQnYNM&feature=player_detailpage#t=101s
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Go to Matter WavesNext PPT
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Einstein realized that matter contains energy. There is an equivalence of mass & energy.Energy is stored in the nucleus of atoms.
The energy stored any mass obeys Einstein’s equation:
E = energy in J.E = mc2. m = mass kg
c = vel of light
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Ex 2: How much energy is produced when 2.5 kg of matter are completely converted to energy?
How much energy is that in eV?
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E = mc2.
=(2.5 kg )(3x108 m/s)2. = 2.25 x 1017 J
in eV
(2.25 x 1017 J)(1 eV / 1.6 x 10 –19 J) = 1.4 x 1036 eV.
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Atomic Mass Units: amu or u
• Mass of atoms very small so they are measured in amu or u.
• Since mass is equivalent to energy,
• 1 amu = 931 MeV or 931 x 106 eV.
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Ex 3: One universal atomic mass unit is equivalent to an energy of 931 MeV. Calculate the mass in kg of one universal mass unit.
Hint: Use E = mc2 where energy is known in eV.
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Don’t forget to convert MeV to eV.
(1 u) x (931 MeV/u) x (106eV/MeV) x (1.6 x 10 –19 J / eV) =
1.49 x 1010 J
E = mc2 so m = E/c2.
(1.49 x 1010 J) / (3x108 m/s)2 =
1.66 x 10 –27 kg
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The mass units are based on the mass of a proton or 1H.
(A hydrogen nucleus)
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Go to “The Nucleus” PPT
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Film: Mech Univ Models of the Atom
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Standard Model:Matter is composed of small subatomic particles called quarks & leptons.
Forces also have particles that transfer information through tiny particles.
See review book xerox.
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Quarks
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Bohr’s model could not explain why e- could occupy only certain orbits.
DeBroglie’s hypothesis for the wave nature of matter helped explain how only certain orbits were allowed.
Each e- has = h/mv.
DeBroglie proposed that each e- is a standing wave.
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Proposed e- standing waves. Only ’s that fit certain orbits are possible.
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’s that don’t fit circumference cannot exist.
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Heisenberg’s uncertainty principle 1927.
It is impossible to be make simultaneous measurements of a particle’s position and momentum with infinite accuracy.
When you try to look to see where an e- actually is, you must give it energy. If you give it energy, it moves.
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Film DeBroglie Atom
http://www.youtube.com/watch?v=IsA_oIXdF_8&feature=iv&annotation_id=annotation_40275
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Alpha Rays
• A rays are helium nuclei, (2p+ and 2no), that are emitted from nucleus.
• They can be easily stopped by skin or thin sheet of paper.
• More likely to knock e- from orbits because they lose all their KE at once.
• Charge = +2e
• Mass 4 units
• Energy is KE = ½ mv2.
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Beta Rays
• More penetrating than alpha.
• Less capable of ionizing because their energy is lost over greater distance.
• They are fast moving e-.
• Charge = -e.
• mass = e.
• KE = ½ mv2. v can be sig portion of c.
• Need a few mm of Al to stop them.
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GammaPenetrating power greatest. Can pass thru human body, concrete, and lead.
Lowest ionizing power.
They are EM waves.
No charge. No mass.
Energy described by E = hf.
Travel with vel of light in vacuum.
No maximum stopping range.
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How could we distinguish the different types of radiation? What could we observe?
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Hwk rd 450 –462 Core only
Do quest pg 451 1-5p 457 1-4p 458 1-3p 462