Isotopes
Isotopes are different forms of an element
having the SAME atomic number, meaning
the same number of protons.
 Isotopes DIFFER in the amount of neutrons.

Isotopes, Radioisotopes and
Atomic Mass
35
37
Cl
17
1
Cl
17
3
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Medical Uses of Isotopes

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Medical Uses of Isotopes
Iodine in nature is 100% iodine-127 (stable)
Iodine-131 is formed in nuclear reactions, it
is unstable and undergoes radioactive decay
 Because these isotopes are absorbed by the
thyroid virtually the same way a doctor can
use in diagnostic test
 Sodium iodide, made with iodine-131, is
administered and instruments are used to
detect radioactive emissions detecting the
levels and locations in patients body
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
Thyroid diseases


The thyroid controls
how quickly the body
burns energy, makes
proteins, and controls
how sensitive the body
should be to other
hormones.
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1
Isotopic Abundance




Different elements have different numbers of
isotopes
These elements exist in different relative
abundances
For example, natural magnesium is a mixture of
three isotopes: Mg-24, Mg-25, and Mg-26
On average, a sample of natural magnesium
consists of 78.7% Mg-24,10.1% Mg-25 and
11.2 % Mg-26
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Average Atomic Mass
Calculating Relative Atomic Mass
The atomic mass of an element is
determined by calculating the weighted
average of the masses of all isotopes of that
element
 A weighted average considered not only the
values but also the relative abundance of
each value

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Calculate the relative atomic mass of chlorine if
the relative abundance of Cl-35 is 75.43%
and Cl-37 is 24.47%.
mCl = (percent abundance of Cl-35 x atomic mass Cl-35) +
(percent abundance of Cl-37 X atomic mass Cl-37)
mCl = (0.7543 X 35 u) + (0.2447 X 37 u)
= 26.40 + 9.05 u
= 35.45 u
Radioisotopes
Table 1: Relative Atomic Masses
Atom
Atomic Mass
Oxygen
16.00 u
Iron
55.85 u
Tin
118.71 u
Aluminum
26.98 u
Nitrogen
14.01 u
Cobalt
58.93 u
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Atoms with an unstable nucleus,
characterized by excess energy imparted to
newly-created radiation particle in nucleus or
to an electron
 Because radioisotopes decay by emitting
nuclear radiation in the form of tiny particles
or energy, therefore they are called
radioactive

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2
Radioisotopes

Radioisotopes
Three main types:
Alpha ( ), Beta ( ), Gamma ( )

Alpha radiation = emission of alpha particles
(2p+, 2n0 same as He nucleus)

Beta radiation = emission of beta particles
(high energy electrons)

Gamma radiation = emission of high energy
electromagnetic waves (have no mass and travel
at the speed of light)
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Radiation have different
penetrating abilities
Radioisotope HALF-LIFE

HALF-LIFE refers to the time for ½ of the
original number of radioactive atoms to decay

Example: The half-life of cesium-137 is 30a.
What mass of cesium-137 would remain from
a 12 gram sample after 30a? After 60a?
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Radioisotope Half-Life
Practice
Radioisotope HALF-LIFE
1 half-life
2 half-lives
30 a
60 a
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Polonium-216 has a half-life of 0.16 seconds.
What mass of Polonium-216 would remain
from a 16 g sample after 0.96 seconds?
0.25 g after 6 half-lives
12 g
6g
3g
Cesium-137
17
18
3