Transcript Lesson_3 Atomic Structure

Atomic Structure
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Atomic and Mass Number

The number of protons in an atom = atomic
number (symbol “Z”)
The mass = protons + neutrons (symbol “A”)

Mass is measured in atomic mass units (µ)
Number of neutrons = mass number – atomic number

Standard Atomic Notation: “Biggie over small”


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Practice
For each example find the
 number of protons
 number of neutrons
 number of electrons
 atomic number
 mass number
A)
19
9
F
B)
C) An element that has 91
protons and 140 neutrons
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Isotopes and
Radioisotopes

Isotopes are different forms of an element
having the SAME atomic number but DIFFER in
the amount of neutrons (and mass).
35
17

Cl
37
Cl
Radioisotopes: Atoms with an unstable nucleus
and decay (or break apart) to produce
radioactive particles

Three types: alpha (α), beta (β), and gamma (γ)
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Medical Uses of Isotopes
Used to detect thyroid
diseases
 Iodine–131
 Body uses it the same
as naturally-occurring
iodine (Iodine-127)
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Other uses of isotopes…
Carbon Dating
(carbon-14)
Smoke Detectors
(americium-241)
Agricultural
Research
(carbon-14,
Phosphorous-32)
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Isotopic Abundance and
Atomic Mass

Different elements have different numbers of
isotopes that exist in different relative
abundances
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Ex: An average sample of magnesium contains:
78.7% Mg-24,10.1% Mg-25 and 11.2 % Mg-26
Atomic mass is the weighted average of the
masses of all isotopes of that element
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Calculating Relative Atomic Mass
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
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Radiation
Three main types: Alpha (), Beta (), Gamma ()

Alpha radiation = 2p+, 2n0 same as He nucleus

Beta radiation = high energy electrons or positrons

Gamma radiation = high energy electromagnetic waves
(have no mass and travel at the speed of light)
Different types of radiation have
different penetrating power
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Radioisotope HALF-LIFE

HALF-LIFE refers to the time for ½ of the
original number of radioactive atoms to decay
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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
1 half-life
2 half-lives
30 a
60 a
12 g
6g
3g
Cesium-137
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Homework
Worksheet + text questions
Old Book

Pg. 29 # 7, 8
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Pg. 32 # 9
New Book
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Pg. 29 # 2, 6, 8
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