# Unit 2: The Atom

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```Unit 2: The Atom
Nuclear Decay
Band Of Stability

Atoms that lie
outside the band of
stability are unstable
 Atoms 1-20 n0/p+ ratio
must be 1:1
 Atoms 21-83 n0/p+
ratio is ~ 1.5 : 1
 Atoms 84 and up are
ALL unstable
Unstable atoms will lose pieces of itself until
it becomes stable. – This is Nuclear Decay
3 Types of Decay
Region A: Atoms have
too many neutrons;
Beta Decay
Region B: Atoms have
too many protons;
Positron emission and
e- capture
Region C: Elements
have too many
neutrons and protons;
Alpha Decay
Alpha Decay



I am an isotope that have too many
neutrons and protons… What to do?
Alpha decay!!!
Atoms that undergo alpha decay lose
2 protons and 2 neutrons
α (alpha particles)are
simply the nucleus of
a He atom with no
electrons!!)
 The result is an atom
with lower atomic
number and mass

4
2
He
Alpha Decay
•Alpha decay is how elements greater than atomic #83
try to become stable.
•They will emit an alpha particle (2 neutrons and 2
protons) to try to become stable.
•Alpha reactions will always have He on the right side!
•To balance: write the upper and lower equations!
210
84
=
=
206
82
+
+
4
2



I am an isotope with too many
neutrons… what to do?
Get rid of a neutron by Beta Decay!
Beta Decay
During Beta decay, a neutron is
converted into a proton and e(which is ejected from the
nucleus)
Neutron = proton + electron

Beta particles are known as high
energy electrons

The extra proton creates an
atom with a higher atomic
number
0
1
B
0 
1
e
There are 2 ways
to write a Beta
particle!
Beta Decay
•Beta decay is
how elements
who have too
many neutrons
try to become
stable (on top of
the band)
•Beta reactions
will always have
ß or e- on the
right side!
14
6
=
=
14
+
7
+
0
-1
Gamma Decay



Gamma decay never happens alone! (it
usually accompanies alpha or beta
decay)
It represents a photon of energy or
light
Gamma decay alone involves no
transformation or change of an atom’s
nucleus
0
0

Gamma Decay
233
233
93
93
=
229 +
4
+
0
Np* Pa He  
229
91
= 91
4
2
+ 2
0
0
+
The * represents extra energy that
will be lost.
0
Practice
For each of the following, fill in the missing parts of the
equation:
I 54 Xe 
131
53
226
88
0
1
131
Ra 
Xe 
222
86 Rn
U  He   
235
92
4
2
0
0
4
2
He
231
90
Th
Nuclear Decay Series
•Radioactive atoms will continue to decay until they
reach a stable nucleus.
•For large atoms
with atomic
number greater
than 83, the
stable element
Nuclear Decay Series
particles can be detected by a
Geiger Counter which gives a clicking
sound for each particle detected.
 Different types of radiation required
different types of protection.
 The greater the distance from a
protection.
Alpha Particles
 Cannot
penetrate very far into matter.
 Cannot penetrate a sheet of paper.
 Only dangerous to humans if eaten or
inhaled.
Beta Particles
 Can
penetrate a bit more deeply
 Can be stopped by a sheet of
aluminum foil or plexiglass.
 Only dangerous to people if eaten or
inhaled.
Gamma Particles
 The
most penetrating (have the most
energy).
 They will penetrate the body through
the skin and do damage.
 Have more energy than an x-ray
 Can only be stopped by thick pieces