Transcript L17-BohrAtom

The Quiz
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Atomic Structure &
Chemistry
Anyone who is not shocked
about quantum theory has not
understood it.
- Nils Bohr
Astronomy Question:
How do we know the composition of a planet,
or extra-solar planet, or star, or galaxy if
we’ve never been there
Nils Bohr (1885-1962) was a Danish
Physicist. His model for the hydrogen
atom helped build quantum theory.
Where we left off
Wait!
1)
2)
3)
4)
An accelerating charged particle emits electromagnetic radiation.
Electrons in a circular orbit about a nucleus are accelerating.
Thus the electrons should emit electromagnetic waves.
Electromagnetic radiation carries away energy (E=hv)
5) Electrons lose energy and should fall into the nucleus
(in less than 1 second!)
You’re a physicist in 1830, maybe working with John Herschel
You decide to try to figure out what happens to an atom when you
give it energy. So…
Let’s excite the atomic
electrons
Let’s excite the atomic
electrons
However, you know more than a physicist in 1830, because you
know that critical result from Max Planck; that is that the energy of
a light particle - a photon - is E=hv, where h=6.626x10-34 J s, is
the Planck constant and v is the frequency of light. Also you know
that v = c/ (because I told you this last class).
Max Planck 1901
A prism separates light into
its wavelength components
However, you know more than a physicist in 1830, because you
know that critical result from Max Planck; that is that the energy of
a light particle - a photon - is E=hv, where h=6.626x10-34 J s, is
the Planck constant and v is the frequency of light. Also you know
that v = c/l (because I told you this last class).
So does a grating
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The Riddle of the Hydrogen
Spectrum
A simple formula defines the wavelengths of the spectral lines of atomic hydrogen.
1 
 1
 10.97 2  2 

m 
n
1
microns
Where  is the wavelength of the emitted light and n and m are integers.
Pick any two integers n and m (n must be less than m), calculate the
wavelength and hydrogen will have a spectral line at that wavelength. No
spectral lines exist at wavelengths not given by this equation.
1 
 1
 10.97 2  2 

m 
n
1
Examples
microns
N = 2, m = 3:
 = [10.97 (1/4 - 1/9) ] -1
=
0.656 microns = 656 nm
N = 2, m = 4:
 = [10.97 (1/4 - 1/16) ] -1 =
0.486 microns = 486 nm
N = 2, m = 5:
 = [10.97 (1/4 - 1/25) ] -1 =
0.365 microns = 365 nm
NOTE: units of length
1 micron = 10-6 meters
1 nm
= 10-9 meters
Bohr’s Atomic Structure
An atom arranges its electrons into
discrete orbits.
These orbits have discrete energy
levels, specific to the element.
For hydrogen, the energy of level n
is: En = -10.97 h c /(n2)
For level m: Em = -10.97 h c /(m2)
When an atom jumps between level
m and level n, where level m has the
highest energy the atom looses
energy equal to Em - En. Since
energy is conserved this energy is
contained in the photon which is
emitted with an energy equal to
E = h c/.
En = -10.97 h c/n2
Em = -10.97 h c/m2
Bohr’s Atomic Structure
Atomic electrons are in discrete orbits.
These orbits have discrete energy levels,
specific to the element.
For hydrogen, the energies of level n is:
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En = -10.97 h c/n2
For level m:
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Em = -10.97 h c/m2
Bohr’s Atomic Structure
When an atom jumps between level m
and level n, where Em > En the atom
looses energy equal to Em - En. Since
energy is conserved this energy is
contained in the photon which is
emitted with an energy equal to E = h
c/l.
Total Energy :
En = -10.97 h c/n2
Em = -10.97 h c /(m2)
Em = -10.97 h c/m2
Bohr’s Atomic Structure
Final Energy = photon energy + atom energy
E = hc/ -10.97 h c /(n2)
Must equal initial or total energy:
-10.97 h c /(m2)
En = -10.97 h c/n2
Ep = hv
Em = -10.97 h c/m2
First 4 shells of a
Hydrogen atom.
Is the quantum
nature of atomic
states a little odd?
A note about QM
Imagine if
potential
energy were
quantized!
Summary
Atoms have discrete energy levels, specific to that
atom.
A photon is absorbed when an electron jumps to a
higher energy level.
A photon is emitted when an electron drops to a
lower energy level.
The emitted/absorbed photon’s energy equals the
difference between the atomic levels involved.
Astronomy Question
How do we measure the composition
of a planet, or extra-solar planet, or
star, or galaxy if we’ve never been
there
Solar Spectrum
Absorption lines appear as hydrogen atoms in the Sun’s atmosphere capture
photons and jump to more excited atomic levels.
Atomic Shells
The discrete electron levels are arranged in
shells. Each shell has a maximum
occupancy.
The first electronic shell can have at most 2
electrons, the second shell has room for 8
electrons and so on.
The 1st shell has the lowest energy. Thus,
elements, in their lowest energy state fill the
1st level first, and then fill the 2nd level next.
These elements are listed in the 1st and 2nd
rows of the periodic table.
Atoms are most stable if their outer shell is
full.
The electrons in outer shells are shielded by
the inner shells from the full attraction of the
nucleus. These electrons participate most
readily in chemical reactions.
Atomic Shells
How many electrons
does neutral Carbon (6
protons) have in its outer
shell?
How many electrons
does neutral Neon (10
protons) have in its outer
shell?
The Periodic Table
Molecular
Sizes
Periodic or Mendelev Table
# protons
1 e- missing to fill shell (Halogens)
1 e- in outer shell (Alkali Metals)
Full outer shell (Noble Gases)
Li: solid, Cs: liquid, Ar: gas, Tc: synthetic
Bonding
There are three major ways that elements bond to form molecules.
Atoms with filled shells, the Noble
Gases, are highly inert.
Atoms with one electron in the outer
shell, and atoms with one electron
missing are, on the other hand,
highly reactive. These atoms form
ionic bonds. The alkali gives up an
electron. The halogen takes the
electron. Elements are bonded by
the electric force between the ions.
Ionic Bonds
Example of ionic bonds
Sapphire
Aluminum oxide, Al2O3
Ruby
Metallic Bonds
Atoms in a metals also
give up electrons,
however the electrons
are not transferred to
the other atom.
Instead, the electrons
are shared by all the
atoms.
The sea of electrons
allow current to flow
through metal. Metals
thus make good
conductors.
In sodium, for example, 1 out of the 11 electrons is
released so that Na has two filled shells. The extra
electrons move around the metal in a “sea” of negative
charge. This negatively charged sea moves around a
regular structure of positive Na ions.
Covalent Bonds
Certain molecules are formed by sharing
electrons. The covalent bond that forms
resembles metallic bonds in that electrons are
shared. Yet, like ionic bonds the electrons are
shared in discrete shells of the atoms and don’t
run willy nilly throughout the material.
Question
How would you covalently bond
two oxygen atoms to make O2?
Oxygen has 8 protons & electrons.
How many electrons would each O
have to share with the other?
Periodic or Mendelev Table
# protons
1 e- missing to fill shell (Halogens)
1 e- in outer shell (Alkali Metals)
Full outer shell (Noble Gases)
What kind of bonding does KBr have?
Answer
Earth’s Atmosphere
Multiple
Covalent Bonds
Gases in Earth’s atmosphere are mainly
covalently bonded molecules or noble gases.
N2
78%
O2
21%
H 2O
0-4%
Ar
0.9%
CO2
0.035%
Ne
0.0018%
He
0.0005%
CH4
0.0001%
H2
0.00005%
O3
0.000004%
Combining C6, N7, O8
Molecular
attractions
Polar molecules are more
positively charged on one
side and more negative
on the other. This
provides a cohesion.
Question
How would you expect KF to be bonded?
Periodic or Mendelev Table
# protons
1 e- missing to fill shell (Halogens)
1 e- in outer shell (Alkali Metals)
Full outer shell (Noble Gases)
What kind of bonding does KBr have?
Summary
Atoms have discrete energy levels, specific to that atom.
A photon is absorbed when an electron jumps to a higher energy level.
A photon is emitted when an electron drops to a lower energy level.
The emitted/absorbed photon’s energy equals the difference between the atomic levels
involved.
Atomic levels can only fit a certain number of electrons (2 in the 1st level, 8 in the 2nd …)
The periodic table is arranged according the electronic shells and the number of
protons/electrons in the atom.
Atoms with filled shells are most stable. Atoms bond in order to achieve this
configuration.
Ionic bonding involves the transfer of electrons from one atom to another.
Covalent bonding involves the sharing of electrons by one or several atoms.
Metallic bonding involves the sharing of electrons by the entire material/metal.
Radioactive Dating
Parent
Carbon-14
Daughter
Nitrogen-14
Half Life
5,730 yrs*
Potassium-40
Argon-40
1.25 billion yrs
Uranium-238
Lead-206
4.5 billion yrs
Thorium-232
Lead-208
14 billion yrs
Rubidium-87
Strontium-87
48.8 billion yrs
Samarium-147
Neodymium-143 106 billion yrs
Uranium-235
Lead-207
704 billion yrs
*Time that it takes wood to have half the C14 of a living plant.
Swisher et al. 1992, Science
Carbon Dating
1. Solar neutrons enter Earth’s atmosphere.
2. Neutrons collide with N14 (7p, 7n),
creating C14 (6p,8n) [n + N14  p + C14 ]
3. Living bodies continually absorb C14 (e.g.
as CO2 in photosynthesis).
4. When the plant or animal dies, it no
longer assimilates C14.
5. The C14 decays (half life of 5730 yrs).
[C14  N14 + e- + ve ] (n  p + e- + ve )
6. The e- emission rate reveals the age.
Measure ages < 70,000 yrs