Transcript Models of the Atom - Red Hook Central School District

Models of the Atom
Models of the Atom
1907 Plum Pudding Model - Thomson
Rutherford Model 1911
Ernest Rutherford “atoms contain a very
small heavy central positive nucleus,
with the eorbiting randomly
around.
Alpha a particles are He nuclei 2p+, and 2no.
2 elementary charges.
Most a particles went straight through, but
the ones that passed closest the Au nucleus
were progressively more deflected.
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 nucleus
by electrostatic F
What kept the neg e- from fall into the nucleus?
-inertia from circular velocity of e- (angular
momentum) balanced the electrostatic attraction
of the nucleus.
+
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?
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.
Angle of Deflection
The a particle repelled straight back would
have to come to rest for a moment. At that
moment its KE would be balanced by
electrical PE.
Angle q of deviation from undeflected path.
Rutherford used scattering angles from many
particles to make his measurement.
q
KE = E elc.
KE = kQq/r
see table p 8 topic 9
V = kq/r.
• Q = charge on nucleus
• q = charge on alpha particle
• r is the “distance of closest approach”
Ex 1: An a 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.
• IB Questions Rutherford.
Bohr proposed working model for H.
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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”
• 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.
• DE = Ef – Ei = hf.
Evidence for Bohr
comes from emission and
absorption spectra of light.
Electric E supplied to gas tubes
causes gases to emit light.
Emission Spectrum
When viewed through a prism or
spectroscope, we see only certain l of light
are emitted by each element.
Bright Line Spectra
Continuous spectrum
From sunlight
Frequencies emitted exactly match the
frequencies absorbed.
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.
Summary
• 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.
Diagrams
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.
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?
Atoms must also absorb energy for the
e- to jump to higher orbits.
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.
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?
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.
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.
Homework Set
• Read Hamper 7.1 pay attention to purple
box. Do 1 – 4 page 149 and
• IB packet Bohr Model prb
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
Go to Matter Waves
Next PPT
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
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?
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.
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.
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.
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
The mass units are based on
the mass of a proton or 1H.
(A hydrogen nucleus)
Go to “The Nucleus” PPT
Film: Mech Univ Models of the
Atom
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.
Quarks
Bohr’s model could not explain why ecould occupy only certain orbits.
DeBroglie’s hypothesis for the wave
nature of matter helped explain how only
certain orbits were allowed.
Each e- has l = h/mv.
DeBroglie proposed that each e- is a
standing wave.
Proposed e- standing waves. Only l’s
that fit certain orbits are possible.
l’s that don’t fit circumference
cannot exist.
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 eactually is, you must give it energy. If you
give it energy, it moves.
Film DeBroglie Atom
http://www.youtube.com/watch?v=Is
A_oIXdF_8&feature=iv&annotation_
id=annotation_40275
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.
Beta Rays
• More penetrating than alpha.
• Less capable of ionizing because their energy is
lost over greater distance.
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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.
Gamma
Penetrating 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.
How could we distinguish the
different types of radiation? What
could we observe?
Hwk rd 450 –462 Core only
Do quest pg 451 1-5
p 457 1-4
p 458 1-3
p 462