Transcript Radioactivity - MrSimonPorter

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)
For example, Lithium atoms occur in two forms, Lithium-6 and
Lithium-7
3 neutrons
4 neutrons
7
6
3
3
Li
Li
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. That’s 100 000 times smaller than
the size of an atom (about 10-10 metres).
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
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.
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)
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
• acts over short distance (10-15 m)
• acts only between adjacent particles in the
nucleus
• is carried by gluons
Unstable nuclei
Some nuclei are unstable, for example
Uranium 235 (it’s to do with the relative
numbers of protons and neutrons)
Hi! I’m uranium-235 and I’m
unstable. I really need to lose
some particles from my
nucleus to become more
stable.
Unstable nuclei
To become stable, an unstable nuclei
emits a particle
Weeeeeeeeeeeeee!
Unstable nuclei
We say the atom has decayed
Weeeeeeeeeeeeee!
Unstable nuclei
The decay of an unstable nucleus is random. We know it’s going to
happen, but we can’t say when! It is spontaneous. It cannot be
affected by temperature/pressure etc.
Weeeeeeeeeeeeee!
Becquerels (Bq)
• The amount of radioactivity given out by a
substance is measured in Becquerels.
One becquerel is one particle emitted per
second.
Detection
• Particles can be detected by photographic
film
• Particles can also be detected (and
counted) by a Geiger-Müller tube (GM
tube) connected to a counter
Alpha particles
•
•
•
•
•
2 protons and 2 neutrons joined together
The same as the nucleus of a helium atom
Stopped by paper or a few cm of air
Highly ionising
Deflected by electric and strong magnetic
fields
2+
4
2
He
Alpha Decay
Atomic mass goes down by 4
235
231
4
92
90
2
U
2+
He
+
Th
Atomic number goes down by 2
Beta particles
•
•
•
•
Fast moving electrons
Stopped by about 3 mm of aluminium
Weakly ionising
Deflected by electric and magnetic fields
0
e
-1
Beta decay
• In the nucleus a neutron changes into an
electron (the beta particle which is ejected)
and a proton (which stays in the nucleus)
• During beta decay the mass number stays
the same but the proton number goes up
by 1.
231
Th
90
antineutrino
Pa + -1e + ‫ע‬e
231
91
0
0
0
Gamma rays
•
•
•
•
High frequency electromagnetic radiation
Stopped by several cm of lead
Very weakly ionising
NOT affected by electric or magnetic fields
Gamma rays
Associated with alpha decay
235
231
92
90
U
Th +
α
½ - life
• This is the time it takes half the nuclei to
decay
Number of
nuclei
undecayed
time
half-life (t½)
½ - life
• This is the time it takes half the nuclei to
decay
Number of
nuclei
undecayed
A graph of the count
rate against time will
be the same shape
time
half-life (t½)
Different ½ - lives
• Different isotopes have different half-lives
• The ½-life could be a few milliseconds or
5000 million years!
Number of
nuclei
undecayed
time
half-life (t½)
Nuclear reactions
14
7
4
2+ 17
2
8
1
N + He O + p
1
Unified mass unit (u)
• Defined as 1/12 of the mass of an atom of
Carbon-12
u = 1.6605402 x 10-27 kg
Energy mass equivalence
• E = mc2
• E = 1.6605402 x 10-27 x (2.9979 x 108)2
• E = 1.4923946316 x 10-10 J
• Remembering 1 eV = 1.602177 x 10-19 J
• 1 u = 931.5 MeV
Mass defect
For helium, the mass of the nucleus =
4.00156 u
But, the mass of two protons and two
nuetrons = 4.0320 u!!!!
Where is the missing mass?
Mass defect
The missing mass (mass defect) has been
stored as energy in the nucleus. It is called
the binding energy of the nucleus.
It can be found from E = mc2
Mass defect calculation
• Find the mass defect of the nucleus of
gold, 196.97 - Au
Mass defect calculation
• The mass of this isotope is 196.97u
• Since it has 79 electrons its nuclear mass
is 196.97u – 79x0.000549u = 196.924u
• This nucleus has 79 protons and 118
neutrons, individually these have a mass
of 79x1.0007276u + 118x1.008665u =
198.080u
• The difference in mass (mass defect) is
therefore 1.156u
Mass defect calculation
• The difference in mass (mass defect) is
therefore 1.156u
• This “missing mass” is stored as energy in
the nucleus (binding energy).
• 1u is equivalent to 931.5 MeV
Binding energy
This is the work required to completely
separate the nucleons of the nucleus.
Binding energy per nucleon
This is the work required to completely
separate the nucleons of the nucleus
divided by the number of nucleons.
It is a measure of how stable the nucleus
is.
The binding energy curve
Nuclear Fission
Uranium
Uranium 235 has a large unstable
nucleus.
Capture
A lone neutron hitting the nucleus can be
captured by the nucleus, forming Uranium
236.
Capture
A lone neutron hitting the nucleus can be
captured by the nucleus, forming Uranium
236.
Fission
The Uranium 236 is very unstable and
splits into two smaller nuclei (this is
called nuclear fission)
Free neutrons
As well as the two smaller nuclei (called
daughter nuclei), three neutrons are
released (with lots of kinetic energy)
Fission
These free neutrons can strike
more uranium nuclei, causing
them to split.
Chain Reaction
If there is enough uranium (critical mass) a
chain reaction occurs. Huge amounts of
energy are released very quickly.
Bang!
This can result in a nuclear explosion!YouTube nuclear bomb 4
Nuclear fusion – Star power!
The binding energy curve