Nuclear Changes

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Nuclear Changes

• Chapter 7

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7.1 What is Radioactivity?

• Large atoms are unstable.
• When the nucleus is crowded with protons and
  neutrons, its just too much.
• The nucleus begins to emit (shoot out) particles
  and/or energy.

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Radioactivity
Penetrating power of different forms of radiation
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Radioactivity
Marie (1867-1934) and Pierre Curie (1859-1906)
isolated polonium and radium from
pitchblende both elements more radioactive
than pure uranium discovered that the source
of energy (radiation) were the atoms
themselves nature of radioactivity was still
unknown
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Radioactivity
Ernest Rutherford (1871-1937)
studied absorption of 'rays' emitted by
uranium-containing minerals two types of rays
?- and ?-rays ?-rays are more penetrating than
?-rays ?- and ?-rays are not rays at all
(like X-rays or light) but streams of particles
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Radioactivity
Ernest Rutherford (1871-1937)
?-particles behave like electrons, (1 negative
charge) - move very fast ?-particles and
have 4 times the mass of a hydrogen nucleus and
twice the charge (2 positive charges)
?-particle Helium nucleus (2 protons, 2
neutrons)
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Radioactivity
?- and ?-radiation are made up of particles,
?-radiation is not! ?-radiation is
electromagnetic radiation (just like light and
X-rays) no mass, no charge
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Radioactivity
Radioactive decay b-decay
Proton
Neutron
Electron
a Neutron may split into a Proton plus an Electron
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Radioactivity
Radioactive decay b-decay
Proton
Neutron
Electron
the electron is ejected from the nucleus as
?-radiation...
...leaving behind a nucleus with an extra proton
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Nuclear vs Chemical Reaction
Chemical Reaction
NaOH HCl ? H2O
NaCl
Na
Cl
Cl
Na
?
H
H
H
H
O
O
Not a true representation of this reaction in
solution
Nuclear Reaction
208
212Po ?
4a 82Pb
2
84
?
Not a true representation of the nuclei
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The Half-Life (t1/2) of a Nuclear Reaction
Half-life (t1/2) The time it takes for half of
the radioactive nuclei in a sample to decay.
of radioactive nuclei
48 radioactive particles at t0
24 radioactive particles at t1 (1 half life)
12 radioactive particles at t1 (2 half life)
6 radioactive particles at t1 (3 half life)
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The Half-Life (t1/2) of a Nuclear Reaction
Half-life (t1/2) The time it takes for half of
the radioactive nuclei in a sample to decay.
of radioactive nuclei
Fraction of nuclei
48 radioactive particles at t0
48/48 1
_at_ t1/2 1 24 1 48 2
24 radioactive particles at t1 (1 half life)
_at_ t1/2 2 12 1 1 1 48 2 2
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12 radioactive particles at t2 (2 half lifes)
_at_ t1/2 3 6 1 1 1 1 48 2 2
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6 radioactive particles at t3 (3 half lifes)
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The Half-Life (t1/2) of a Nuclear Reaction
Half-life (t1/2) The time it takes for half of
the radioactive nuclei in a sample to decay.
of radioactive nuclei
Fraction of nuclei
General Formula Fraction remaining 1
2n where n is the of
half lifes
48 radioactive particles at t0
48/48 1
_at_ t1/2 1 24 1 48 2
24 radioactive particles at t1 (1 half life)
_at_ t1/2 2 12 1 1 1 48 2 2
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12 radioactive particles at t2 (2 half lifes)
_at_ t1/2 3 6 1 1 1 1 48 2 2
2 8
6 radioactive particles at t3 (3 half lifes)
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Lets go over all that again!
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Phenomenon of Radioactivity
Some elements, such as uranium (U) and thorium
(Th), are unstable They decay spontaneously.
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Uranium Nucleus
spontaneously emits a particle from its
nucleus called an alpha particle (2 protons 2
neutrons).   
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Alpha Particle
emits a particle from its nucleus
called an alpha particle (2 protons 2
neutrons).   
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Uranium - Thorium Decay
spontaneous
decay
parent
daughter product
alpha particle 2 protons 2 neutrons
positively charged ion of Helium
Thorium 90 protons 144 neutrons
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Beta Particle Emission
But, Th is also unstable, and it emits a beta
particle
234
90
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Thorium - Protactinium Decay
beta particle
Th Pa
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234
90
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beta particle an electron discharged from the
nucleus when a neutron splits into a proton and
an electron
Protactinium 91 protons 143 neutrons
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Title
beta particle an electron discharged from the
nucleus when a neutron splits into a proton and
an electron
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U PbSeries
This process is called radioactive decay, and
eventually uranium (parent) decays to lead
(daughter product).
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U PbSeries
The rate at which this process occurs is measured
in terms of the half life.
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Half Life
Half Life Number of years for 1/2 of the
original number of atoms to decay from U to Pb
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Carbon-14 Dating
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