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IsotopesAverage Atomic Mass
Isotopes Atoms of the same element (same atomic number)
with different mass numbers
Atoms with the same number of protons, but different
numbers of neutrons.
Isotopes of chlorine
35Cl 37Cl17 17
Chlorine - 35 Chlorine - 37
Facts About Isotopes
Almost each element is found in the Universe as a mixture of isotopes
Every isotope is found in nature in a fixed percentage called Isotopic Abundance
For example: Magnesium can be found as a mixture of three isotopes 79% Mg-24, 10% Mg-25 and 11% Mg-26
Facts About Isotopes These percentages are different for
each element Each element has a particular number
of isotopes The isotope notation includes the name
or symbol of the element followed by the atomic mass.
Example: Magnesium-25 or Mg-25
The existence of isotopes explains the fact that atomic mass is a decimal for most elements
Isotopes of one element have the same chemical behaviour but slightly different physical properties (e.g. boiling point, melting point)
They have lots of real life applications (medical field, agriculture,..)
There are almost 2000 isotopes found The isotopes of radioactive elements are
called Radioisotopes
Learning Check Naturally occurring carbon consists of three
isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.
12C 13C 14C 6 6 6
#p _______ _______ _______
#n _______ _______ _______
#e _______ _______ _______
Atomic Mass on the Periodic Table
11
Na22.99
Atomic Number
Symbol
Atomic Mass
Atomic Mass
Atomic mass is the weighted average mass of all the atomic masses of the isotopes of that atom correlated with their isotopic abundances.
where: Aav = average atomic mass (%) = isotopic abundance
nnav AAAA (%)...........(%)(%) 2211
Example 1: Average Atomic Mass
Cl-35 is about 75.5 % and Cl-37 about 24.5% of natural chlorine.
Calculate the average atomic mass for Chlorine.
Example 2: Average Atomic Mass
Assume that the element 108Uno is synthesized and that the sample contains 25.2% 272Uno (271.4 amu), 30.8% 273Uno (272.3 amu) and 44.0% 275Uno (274.2 amu).
What is the value of the atomic weight?
Calculating Isotopic Abundance
Steps: Assign variables for the isotopic
abundances; x and y Write the eq. for isotopic abundances: x + y = 1 Write the eq. for the Average atomic
mass: Set both eq. as a linear system and solve
for x and y by substitution
yAxAAav 21
Practice!
Boron exists as two naturally occurring isotopes:
B-10.01a.m u and B-11.01a.m.u. Calculate the isotopic abundance of
each isotope of Boron.
Practice!
Silver (Atomic weight 107.868) has two naturally-occurring isotopes with isotopic weights of 106.90509 and 108.90470.
What is the percentage abundance of each isotope in Silver?
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