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Radioactivity Topic
Many atoms have an unstable nucleus. Such atoms are said to be radioactive and will
undergo decay.
All radioactive isotopes (radioisotopes) will turn into stable atoms by decaying, but as
they do so they give out radiation.
Radiation consists of 3 types of particles:
Summary table
Particle Description How it
affects
the atom
Alpha A Helium When an
nucleus
alpha
Symbol
4
particle
He
is lost the
2
atoms
Contains: mass no.
2 protons decreases
2 neutrons by 4 and
the
atomic
no.
decreases
by 2.
What stops
it
Skin or
paper
It has little
penetrating
power.
Beta
This is a
fast moving
electron.
Just like an
electron it
has no mass
but a charge
of -1
When a
beta
symbol
particle is
lost the
atoms
mass
stays the
same.
The
atomic
no.
increases
by 1.
Gamma High energy A gamma
rays
rays has
symbol
no affect
on mass
or atomic
no.
Examples of Radioactive equations:
ALPHA DECAY
Thin sheets
of
aluminium
metal
(mm) thick
Travels
several
metres.
Only
stopped by
thick sheets
of lead or
concrete.
241

Am
+ Alpha particle
95
Complete the equation:
241

Am
95
237
Np +
93
4
He
2
NOTE: THE ATOMIC NO.
DECREASES BY 2 AND MASS NO.
DECRESES BY 4.
# Note both sides of the equation are
equal: mass numbers and atomic
numbers add up.
BETA DECAY
14
C
6

+ Beta particle
Therefore:
14
C
6
14

N +
7
0
e
-1
NOTE: THE ATOMIC NUMBER
INCRESES BY 1
Complete the following:
What is formed when an alpha
particle is given off from?
239
Pu 
94
238
U
92
232
Th
90
What is formed when a beta particle is
given off from?
234
Th
90
212
Pb
82
131
I
53
Complete questions 1-3 page 209
Half Life
Definition: This is the time taken for a
radioactive substance to decay to half
its original value.
Example: If the half life for A is 24
days. How long will it take to decay to
6.25 %
100% A
50% A
25% A
12.5% A
There are 4 half life values to get from
100%- 6.25%
6.25% A
4 X 24 days = 96 days
Questions:
1. Find the time taken for B to decay to 12.5% if the half life is 27 hours?
2. What is the half life if the time taken to decay to 1/16th of its orginal
value is 128 years?
Summaries
radiation
What is nuclear fission
the
uses
of
ionising