The Electromagnetic Spectrum
7.1 Atomic Theory and
Radioactive Decay
Natural background radiation exists all around
us.
 Radioactivity is the release of high energy
particles or waves

– When atoms lose high energy particles and waves,
new atoms can be formed.
– Radiation is useful but it damages DNA in cells:
– Early discoveries of radiation relied on photographic
equipment
 Marie Curie and her husband Pierre
named the energy radioactivity
 X-rays, radiation therapy and electricity generation
Radium salts, after being placed on a
photographic plate, leave behind the dark
traces of radiation.
Isotopes

39
19
K,
40
19
K,
41
19
Representing Isotopes
K
are atoms of the same element, with a different
number of neutrons in the nucleus.
– changing the # of neutrons changes the mass number
 Remember: mass # = # protons + # neutrons
– isotopes still have the same number of protons and the
same element symbol

Isotopes are written using standard atomic notation
Ex.
Mass
number
Atomic
number
39
19
40
19
K
K,
40
19
K,
41
19
K
Atomic Mass (the decimal #’s)

Atomic mass = average of the mass numbers for
all isotopes of an element.
19
19
19
20
21
22
19
19
19
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Uranium goes through many decay steps
before it becomes stable:
Radioactive Decay

Can result in new atoms forming.
– Radioactivity results from having an unstable
nucleus.
– Radioactive decay = when nuclei break apart +
release energy from the nucleus.
 Radioactive decay continues until a stable element
forms.
 An element may have isotopes that are radioactive
called radioisotopes
– Ex. carbon-12, carbon-13 and carbon-14
(only C-14 is radioactive)

Rutherford identified three types of radiation
using an electric field.
– Positive alpha particles were attracted to the negative plate.
– Negative beta particles were attracted to the positive plate.
– Neutral gamma radiation did not move towards any plate.
Alpha Radiation:

is a stream of alpha particles, (shown as )
– positively charged
– weighs the most
– the same as a helium nucleus
– Alpha particles are represented by the symbols
4
2
 or 42 He
 2 protons and 2 neutrons make a mass number of 4
 it has a charge of 2+ because of the protons
2
Beta Radiation:
Alpha particles are big and slow. A sheet of paper
will stop an alpha particle.


Example: the alpha decay of Radium - 226
A Beta particle, , is a high energy electron.
– negatively charged, and weigh less than alpha
particles.
– Beta particles are represented by the symbols
0
-1
226
88

Ra 
222
86
Rn + 24 
or
226
88
Ra 
222
86
Beta decay occurs when a neutron changes into a
proton + an electron.
131
53
I 
131
54
Xe +
0
–1

or
131
53

I 
131
54
Xe +
0
–1
It takes a thin sheet of aluminum foil to stop a
beta particle.
e
e
Gamma Radiation:

Example: The beta decay of iodine - 131
0
-1
 electrons are very tiny, so beta particles are assigned a
mass of 0.
 one electron gives a beta particle has a charge of 1–.
Rn + 24 He
– The proton stays in the nucleus, and the electron is
released.
 or
Gamma radiation, , is a ray of high energy, shortwavelength radiation.
– has no charge and no mass.
– is the highest energy form of electromagnetic radiation.
– It takes thick blocks of lead or concrete to stop gamma
rays.
– Gamma decay results from energy being released from
a high-energy nucleus.
60
28
Ni* 
60
28
Ni + 00
Shows unstable nucleus for
gamma decay
3
Nuclear Equations:


Often, other kinds of radioactive decay will
also release gamma radiation.
238
92
U 
– Chemical equations represent changes in the
position of atoms, not changes to the atoms
themselves.
Th + He + 2
234
90
are written like chemical equations, but
represent changes in the nucleus of atoms.
4
2
– Uranium-238 decays into an alpha particle and
also releases gamma rays.

Remember:
1.The sum of the mass numbers on each side of
the equation should equal.
2.The sum of the charges on each side of the
equation should equal.
Summary Tables
7.2 Half-Life

the time it takes for half of a radioactive
sample to decay
– is a constant rate (always the same half life for
each element)
Example: Strontium-90 has a half-life of 29 years. If
you have 10 g of strontium-90 today, there will be
5 g (half) remaining in 29 years.
Terminology:
Parent isotope: the original radioactive material
Daughter isotope: the stable product that remains
after decay has happened
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Radioactive Dating:
Decay Curves:
a method to determine age of objects
 compares amount of parent isotope to
daughter isotope.
Example: Carbon dating measure the ratio of
carbon-12 and carbon-14.

Show the rate of decay
for radioactive elements
 show the relationship
between half-life and
percentage of original
substance remaining.

– Stable carbon-12 and radioactive carbon-14
exist naturally in a constant ratio.
 In nature, carbon-12 appears 98.9% of the time,
while carbon-14 is very rare.
There are many radioisotopes that
can be used for dating

When an organism dies, carbon-14 stops being
taken in and continues to slowly decay.
– Comparing the amounts of carbon-12 to carbon-14
is called radiocarbon dating and gives us an age for
the object.
– Radiocarbon dating only works for organisms less
than 50,000 years old because the half-life of
carbon-14 is 5730 years. (after 50,000 years, there
isn’t enough C-14 left!)
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The Potassium-40 Clock

some elements require one step to decay,
while others decay over many steps before
reaching a stable daughter isotope.
– Carbon-14 decays into nitrogen-14 in one step
– Uranium-235 decays into lead-207 in fifteen steps.
– Thorium-235 decays into lead-208 in ten steps.

The potassium-40/argon-40
clock has a half-life of 1.3
billion years.
 Argon-40 produced by the
decay of potassium-40
becomes trapped in rock.
 Ratio of potassium-40 : argon40 shows age of rock.

Half-life Problems:
1. What mass of a 200g sample of carbon-14
remains after 22,920 years?
2. A rock has 420g of radioactive isotope.
What percentage would remain after 5
half-lives?
Radioisotopes with very long half-lives can
help determine the age of very old things.
7.3 Nuclear Reactions:

Nuclear reactions are different than chemical
reactions
Chemical Mass is
Reactions conserved
(doesn’t
change)
Nuclear Small
Reactions changes in
mass
Small
energy
changes
No changes in the
nuclei
Huge
energy
changes
protons, neutrons,
electrons and
gamma rays can be
lost or gained
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Induced Nuclear Reactions

Symbols to Remember:
Scientists can also force ( = induce) nuclear
reactions by smashing nuclei with alpha, beta and
gamma radiation to make the nuclei unstable
+
4
2
14
7
N 
17
8
O + 11 p
17
8
O + 11 H
or
4
2
He +
14
7
N 
Nuclear Reactions
Two types:
– Fission = the splitting of nuclei
– Fusion = the joining of nuclei (they
fuse together)
 Both reactions involve extremely large
amounts of energy

Albert Einstein’s
equation E = mc2
illustrates the energy
found in even small
amounts of matter
Nuclear Fission:
is the splitting of one heavy nucleus into
two or more smaller nuclei, as well as some
sub-atomic particles and energy.
 A heavy nucleus is usually unstable, due to
many positive protons pushing apart.
 When fission occurs:
1.Energy is produced.
2.More neutrons are given off.

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Induced Nuclear Fission

Neutrons are used to make nuclei unstable
Induced Nuclear Fission
of Uranium-235

is the origin of nuclear power and nuclear bombs.
– It is much easier to crash a neutral neutron than a positive
proton into a nucleus to release energy.
1
• A neutron, 0 n, crashes into an atom of stable
uranium-235 to create unstable uranium-236, which
then decays.
• After several steps, atoms of krypton and barium are
formed, along with the release of 3 neutrons and
huge quantities of energy.
Chain Reactions:

The neutrons released in the induced
reaction can then trigger more reactions on
other uranium-235 atoms…causing a CHAIN
REACTION
– A chain reaction can quickly get out of control
 materials that absorb some neutrons can help to control
the chain reaction.
– Nuclear reactors have complex systems to ensure the
chain reaction stays at safe levels.
– An uncontrolled chain reaction can result in the
release of excess energy as harmful radiation
 It is on this concept that nuclear bombs are created.
 Nuclear “meltdown” occurs if the chain reactions cannot be
controlled
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Hazardous Wastes
Nuclear Energy

Nuclear power plants can generate large
amounts of electricity using fission reactions.
– In Canada, Ontario, Quebec and New Brunswick
currently use nuclear power.
 Canadian-made nuclear reactors
are called CANDU reactors.
 CANDU reactors are considered
safe and effective, and are sold
throughout the world.
Nuclear Fusion

joining of two light nuclei
into one heavier nucleus.
– In the core of the Sun, two
hydrogen nuclei join under
tremendous heat and pressure
to form a helium nucleus.
– When the helium atom is
formed, huge amounts of
energy are released.

Hazardous wastes produced by nuclear
reactions are problematic.
– Some waste products, like fuel rods, can be re-used
– Some products are very radioactive, and must be
stored away from living things.
 Most of this waste is buried underground, or stored in
concrete
 It takes 20 half-lives (thousands of years) before the
material is safe.
The fusion of
hydrogen
nuclei

Scientists cannot yet find a safe, and
manageable method to harness the energy
of nuclear fusion.
– “cold fusion” would occur at temperatures and
pressures that could be controlled (but we
haven’t figured out how to get it to happen)
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