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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 For each example find the number of protonsnumber of protons number of neutronsnumber of neutrons number of electronsnumber of electrons atomic numberatomic number mass numbermass number
F19
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A)
B)
C) An element that has 91 An element that has 91 protons and 140 protons and 140 neutronsneutrons
Isotopes and Radioisotopes
Isotopes are different forms of an element having the SAME atomic number but DIFFER in the amount of neutrons (and mass).
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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Cl35
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37Cl
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…
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Carbon Dating(carbon-14)
Smoke Detectors(americium-241)
Agricultural Research
(carbon-14, Phosphorous-32)
Isotopic Abundance and Atomic Mass
Different elements have different numbers of isotopes that exist in different relative abundances 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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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
Calculating Relative Atomic Mass
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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)
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Different types of radiation have different penetrating power
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
Cesium-137
12 g
2 half-lives
60 a
1 half-life
30 a
6 g 3g
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Homework
Worksheet + text questions
Old Book
Pg. 29 # 7, 8
Pg. 32 # 9
New Book
Pg. 29 # 2, 6, 8
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