Transcript Atomic Models and Radioactivity - Cashmere

Atomic Models and
Radioactivity
NCEA AS 2.5
Text Chapter: 14
History

Greeks:
4 types of atoms, earth, air, fire, water
Used these atoms to explain why things
happened
Eg stones fell to the earth because they
were made of earth atoms
Atomos = “indivisible”
History
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Early 1800s
John Dalton, an observer of weather and
discoverer of color blindness among other
things, came up with atomic theory
All matter is made up of small indivisble
particles known as “atoms”
Atoms were solid spheres
Drew the first molecular diagrams
History
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J.J. Thompson (1856 -1940)
Studied the “mysterious cathode rays
In 1903 he proposed the “Plum pudding
model” for the atom
the atom is a sphere of positively charged
matter with electrons embedded like the
currents in a “plum pudding”
Thompson’s Model
History
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Ernest Rutherford
Famous for his gold foil experiment
Atom is mainly empty space
Small dense positively charged nucleus
Electrons orbiting the nucleus
(This is the model you have to be able to
explain for this achievement standard)
Since Rutherford….
With the help of quantum theory that was being
developed by Planck, Einstein and others, the
model continued to evolve…
 Neils Bohr (1913): electrons occupy fixed
energy levels (not fixed positions)
 Louis de Broglie (1924): electrons are waves
 Erwin Schrodinger (1925): electrons are matter
waves whose position is based on a statistical
probability (enter quantum mechanics)
 Chadwick (1935): Discovers the neutron.
Rutherford’s Gold Foil Experiment

He fired alpha
particles at a very thin
piece of gold foil and
measured the angles
they were scattered
at.
The Results
Observation 1
Most passed right through the gold foil
Explanation 1
Atoms are mostly empty space
The Results
Observation 2
Some were deflected
Explanation 2
The atom contains a positive charge in its
centre or nucleus that deflects
alpha particles (which are positively charged)
The Results
Observation 3
A rare few bounced directly backwards
Explanation 3
The positive charge must be small and densely
packed so only a few alpha particles hit it
directly head-on and bounce back
Rutherford’s Model

Positive nucleus
surrounded by
orbiting negatively
charged electrons
The Nucleus
Writing nuclei
X = element symbol
A = mass number or nucleon
number (the number of p+n)
Z = atomic number (the
number of protons)
How many of each sub-atomic
particle does potassium have?

A
Z
X
39
K
19
Isotopes
Atoms with the same atomic number but
different mass numbers
 Eg:
12
1
6
1
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C
H
2
1
3
1
H (deuterium )
13
6
H (tritium )
14
6
C
C
Just Interesting
In 1896 Henri Becquerel discovered by
chance that a rock containing uranium
when left on wrapped photographic paper
was able to darken it.
 Pierre and Marie Curie, French chemists
did most of the early work on radio active
substances. They discovered radium
which is far more active than uranium
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Radioactivity
Radioactivity is a natural decay process
Radioactivity is unaffected by external
conditions like temperature
Radioactivity involves the nucleus of the
atom
Some elements spontaneously emit
particles or energy from their nucleus
In a nuclear reaction atoms can change
from one element to another.
Nuclear Reactions
3 types:
Radioactive Decay – the spontaneous
emission of particles from the nucleus of
an atom
Nuclear Fission – splitting one large
nuclei into two smaller ones
Nuclear Fusion – combining two small
nuclei into one large one.
Radioactivity

3 types:
Alpha a
 Beta b
 Gamma g

Named in order of their discovery.
 Alpha and beta decay don’t usually occur by
themselves, there is usually some gamma
that occurs with them.
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The Nucleus
In small atoms, the number of protons and
neutrons are usually the same (roughly)
 In larger atoms, there are usually many
more neutrons than protons, in order to
keep the nucleus stable.
 If a nucleus is unstable, it may
spontaneously decay to something more
stable by emitting alpha, beta or gamma
radiation

Alpha Particles
Helium nucleus
 Charge of +2
 Mass of 4 (a.m.u)
 Travel slowly ie. 10% of light speed
 Don’t travel very far ie. A few cms in air
 Low penetration power – can be stopped
by a piece of paper
 Very good ionising power – because
they’re big and slow.
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Beta Particles
An electron from the nucleus
 Charge of -1
 Same mass as an electron (effectively 0)
 Travel relatively fast – up to 95% of light
speed
 Travel about 30 cms in air
 Average penetration power – can be
stopped by a few mm of Aluminium
 Average ionising power
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Gamma Radiation
A wave of electromagnetic radiation
(energy)
 No charge
 No mass
 Travels at light speed
 Travels several metres in air
 High penetration power – Several cms of
lead needed to stop it
 Low ionising power – because no mass
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Radiation
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One way that the different types of
radiation can be distinguished is by
observing their behaviour in a magnetic
field:
b
g
a
Balancing Nuclear Equations
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The atomic numbers are conserved.
That means they have to add up to the same
number on both sides of the equation.
The same applies to the atomic masses.
I  Xe 
131
53
131
54
0
?
1
b
Note: Both mass and charge must be conserved
Try this
226
88
Ra  Rn  He  g
222
86
4
2
(ie 226=222+4, 88=86+2)
Another One
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Cobalt 60 decays to Nickel 60 emitting a
a gamma ray and ?
Co Ni e  g
60
27
60
28
0
1
Again,
mass and charge are conserved
NB. the a or b symbols can be used instead of
He or e
Why have 2 different atoms got the same
atomic mass? (type 1 question)
Last one for now
1
0
n  p e
1
1
0
1
That’s how the nucleus is able to emit
electrons
What is bigger a proton or neutron?
(Type 1 question again)
Think about it and don’t yell out
Half-life
The time it take for the decay rate to have
halved, or….
 The time taken for half of the original
atoms to have decayed
 Usually shown on a graph
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Half-life
Half Life
4500
4000
No. of Atoms
3500
3000
2500
2000
1500
1000
500
0
0
1
2
3
Time in days
What ½ is being shown here?
4
5
Answer These
The half-life of iodine-128 is 25 minutes.
How long would it take the radioactivity of a
sample to drop from…………….
 3000 decays per second to 750 decays per
second
 2 half lives 50minutes
 3000 to 375 decays per second
 3 half lives 75 minutes
Answer These
The half life of thorium-234 is 24 days.
Initially a sample contains 6.4 x 1020 atoms.
How many thorium atoms will remain after
72 days?
 8 x 1019 atoms
How long will it take for 6.3 x 1020 thorium
atoms to decay.
 144 days
Are studying for 2.7 yet?????
The background count in S2 averages
out at 6 min-1 .
If a radio active source has
a count of 246 min-1 and a week
later 21 min-1 what is the ½ life?
24015
4 x ½ lives
42 hours
A sample of atoms 16.5x109
has a 168 hour ½ life.
How many decayed atoms
are there 3 weeks later?
16.5x109 – (16.5x109 x ½ x ½ x ½) =
14.4 x 109
A radioactive sample has a ½ life of 3.6 days
and 1.5 g of radioactive sample remain.
How much was there 14.4 days ago?
14.4/3.6=4
So 4 half lives ago
1.5x2x2x2x2= ?
24g
Detecting Radioactivity
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Geiger Counter – detects electrical current
caused by the ionisation of atoms in a gas
Geiger-Muller
tube filled with
low pressure Ar
End: thin mica
window
+Cathode: metal cylinder
400V DC
Supply
- Anode: central wire
Counter or
speaker
Uses of Radioactivity
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Radiation therapy to treat cancer - g
Power stations - 24a, -10b, g
Sterilization - g
Carbon dating - -10b
Nuclear medicine eg tracers - -10b, g
Smoke detectors - 24a
Detecting leaks in metal pipes - g
Blowing up your neighbors (not)
Nuclear Fission
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1
0
Breaking large unstable nuclei into smaller ones.
Releases a lot of energy
Lots of possible combinations of fragments from
one initial nucleus
Eg:
n U  Ba  Kr  3 n
235
92
141
56
92
36
1
?
0
n
Nuclear Fission
U
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Only one
neutron is
needed to
start the
reaction, but
several are
produced
This starts a
“chain
reaction”
Kr
Ba
n
Kr
Ba
n
n
U
U
U
n
n
n
Kr
Ba
n
n
n
Kr
Ba
n
n
n
Nuclear Fission
If the chain reaction is controlled it can be
used in a nuclear reactor
 If it is uncontrolled it explodes as a nuclear
or atomic bomb
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Nuclear Fusion
The joining of two small nuclei to form one
larger one
 Again, a lot of energy is produced
 This is the process that powers the sun
 Eg:
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2
1
H H 
3
1
4?
2
He
 n
1
0
Nuclear Fusion
Fusion requires extreme temperature and
pressure to occur, and has not practically
and economically been used in power
generation (yet….)
 Hydrogen bombs have been successfully
made, but require a fission reaction to
provide the necessary temp and pressure.
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