Transcript Lesson 1 Rutherford and Bohr Model

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MOODOMETER
Radioactivity
The atom
orbiting electrons
Nucleus (protons
and neutrons)
Nuclide notation
Nucleon number (A) =
number of protons and
neutrons
Neutron number (N) = A - Z
7
Li
3
Proton number (Z) = number of
protons
Isotopes
It is possible for the nuclei of the same element
to have different numbers of neutrons in the
nucleus (but it must have the same number of
protons)
7
6
3
3
Li
Li
Isotopes
For example, Lithium atoms occur in two forms,
Lithium-6 and Lithium-7
4 neutrons
3 neutrons
7
6
3
3
Li
Li
Isotopes of Hydrogen
The three isotopes of Hydrogen even have their
own names!
Hi! I’m
hydrogen
Hola! Mi
nombre es
tritium y yo
soy de
Madrid!
They call
me
deuterium
1
2
3
1
1
1
H
H
H
How do we know the structure
of the atom?
The famous Geiger-Marsden Alpha
scattering experiment
In 1909, Geiger and Marsden were studying how
alpha particles are scattered by a thin gold foil.
Thin gold foil
Alpha
source
Geiger-Marsden
As expected, most alpha particles were
detected at very small scattering angles
Thin gold foil
Alpha particles
Small-angle
scattering
Geiger-Marsden
To their great surprise, they found that
some alpha particles (1 in 20 000) had
very large scattering angles
Thin gold foil
Alpha particles
Large-angle
scattering
Small-angle
scattering
Explaining Geiger and Marsdens’ results
The results suggested that the positive (repulsive) charge must be
concentrated at the centre of the atom. Most alpha particles do not pass
close to this so pass undisturbed, only alpha particles passing very close to
this small nucleus get repelled backwards (the nucleus must also be very
massive for this to happen).
nucleus
Rutherford did the calculations!
Rutherford (their supervisor) calculated
theoretically the number of alpha particles
that should be scattered at different angles.
He found agreement with the experimental
results if he assumed the atomic nucleus
was confined to a diameter of about 10-15
metres.
Rutherford did the calculations!
That’s 100 000 times smaller than the size of
an atom (about 10-10 metres).
Stadium as atom
YouTube - Structure of the Atom 3: The Rutherford Model
If the nucleus of an atom was a ping-pong
ball, the atom would be the size of a football
stadium (and mostly full of nothing)!
http://www.youtube.com/watch?v=XBqHkraf8iE
Nucleus
(pingpong ball
Limitations of this model?
• According to the theory of
electromagnetism, an accelerating charge
(and the orbiting electrons ARE
accelerating centripetally) should radiate
energy and thus spiral into the nucleus.
Evidence for atomic energy levels
Evidence for atomic energy levels
When a gas is heated to a high
temperature, or if an electric current is
passed through the gas, it begins to glow.
Light emitted
cathode
Low pressure gas
anode
electric current
Emission spectrum
If we look at the light emitted (using a
spectroscope) we see a series of sharp
lines of different colours. This is called an
emission spectrum.
Absorption Spectrum
Similarly, if light is shone through a cold gas,
there are sharp dark lines in exactly the same
place the bright lines appeared in the
emission spectrum.
Light
source
gas
Some wavelengths missing!
Why?
Scientists had known
about these lines
since the 19th century,
and they had been
used to identify
elements (including
helium in the sun), but
scientists could not
explain them.
Niels Bohr
In 1913, a Danish
physicist called Niels
Bohr realised that the
secret of atomic
structure lay in its
discreteness, that
energy could only be
absorbed or emitted
at certain values.
At school they
called me
“Bohr the
Bore”!
The Bohr Model
Bohr realised that the
electrons could only
be at specific energy
levels (or states)
around the atom.
The Bohr Model
We say that the
energy of the electron
(and thus the atom)
can exist in a number
of states n=1, n=2,
n=3 etc. (Similar to
the “shells” or
electron orbitals that
chemists talk about!)
n=1
n=2
n=3
The Bohr Model
The energy level diagram of the hydrogen
atom according to the Bohr model
Energy
eV
0
High energy n levels are very
close to each other
n=5
n=4
n=3
n=2
Electron can’t have less
energy than this
-13.6
n = 1 (the ground state)
The Bohr Model
An electron in a higher state than the ground state is
called an excited electron.
Energy
eV
0
High energy n levels are very
close to each other
n=5
n=4
n=3
electron
n=2
-13.6
n = 1 (the ground state)
Atomic transitions
If a hydrogen atom is in an excited state, it can make a transition to
a lower state. Thus an atom in state n = 2 can go to n = 1 (an
electron jumps from orbit n = 2 to n = 1)
Energy
eV
0
n=5
n=4
Wheeee!
n=3
electron
n=2
-13.6
n = 1 (the ground state)
Atomic transitions
Every time an atom (electron in the atom) makes a transition, a
single photon of light is emitted.
Energy
eV
0
n=5
n=4
n=3
electron
n=2
-13.6
n = 1 (the ground state)
Atomic transitions
The energy of the photon is equal to the difference in energy (ΔE)
between the two states. It is equal to hf. ΔE = hf
Energy
eV
0
n=5
n=4
n=3
electron
n=2
ΔE = hf
-13.6
n = 1 (the ground state)
The Lyman Series
Transitions down to the n = 1 state give a series of
spectral lines in the UV region called the Lyman series.
Energy
eV
0
n=5
n=4
n=3
n=2
-13.6
n = 1 (the ground state)
Lyman series of spectral lines (UV)
The Balmer Series
Transitions down to the n = 2 state give a series of
spectral lines in the visible region called the Balmer
series.
Energy
eV
0
n=5
n=4
n=3
n=2
Balmer series of spectral
lines (visible)
-13.6
n = 1 (the ground state)
UV
The Pashen Series
Transitions down to the n = 3 state give a series of
spectral lines in the infra-red region called the Pashen
series.
Energy
eV
0
n=5
n=4
n=3
Pashen series (IR)
n=2
visible
-13.6
n = 1 (the ground state)
UV
Emission Spectrum of Hydrogen
The emission and absorption spectrum of hydrogen is
thus predicted to contain a line spectrum at very
specific wavelengths, a fact verified by experiment.
Which is the emission spectrum and which is the
absorption spectrum?
Pattern of lines
Since the higher states are closer to one another, the wavelengths
of the photons emitted tend to be close too. There is a “crowding” of
wavelengths at the low wavelength part of the spectrum
Energy
eV
0
n=5
n=4
n=3
n=2
Spectrum produced
-13.6
n = 1 (the ground state)
How do you excite an atom?
1. Heating to a high
temperature
2. Bombarding with
electrons
3. Having photons
fall on the atom
I’m excited!
Limitations of the Bohr Model
1. Can only treat atoms or ions with one
electron
2. Does not predict the intensities of the
spectral lines
3. Inconsistent with the uncertainty principle
(see later!)
4. Does not predict the observed splitting of
the spectral lines
Forces in the nucleus
The Coulomb Force
• The repulsive force between protons in the
nucleus
+
+
The Strong Force
The nucleons (protons and neutrons) in
the nucleus are bound together by the
strong nuclear force
The Strong Force
• acts over short distance (10-15 m)
• acts only between adjacent particles in the
nucleus
• is carried by gluons
Questions!
Page 372,
Questions 1, 4,
6, 8, 10, 11.