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Relative Atomic Mass (Ar)
Relative Atomic Mass (Ar)
Since not all atoms of an element have the same number of neutrons, atoms of the same element can have different atomic masses.
Take chlorine, with 2 isotopes: 35Cl and 37Cl.
Why is the atomic mass for Cl on the periodic table 35.45, rather than 35 or 37?
Isotopes do not naturally exist in equivalent proportions; in other words, there is not 50% Cl-35 and 50% Cl-37.
Natural abundance (or percent abundance) is the percentage of a particular isotope that occurs in nature.
Percent abundance factors into determining the relative atomic mass of an element—a weighted average.
Relative atomic mass (Ar) is the average mass of all the isotopes of an element.
Example: The two isotopes of chlorine occur roughly in the ratio of 3:1. So a naturally occurring sample of chlorine contains 75% of Chlorine-35 and 25% of Chlorine-37. Determine the relative atomic mass of chlorine.
Practice 1:
Practice 1:
The three isotopes of magnesium have the following natural abundances:
Calculate the relative atomic mass of Mg.
Ar = (24.0 x 78.70) + (25.0 x 10.13) + (26.0 x 11.17) / 100
Ar = 24.31
Practice 2:
Practice 2:
Chlorine has 2 stable isotopes—Chlorine-35 and Chlorine-37. Calculate the exact natural abundance of each isotope, given an Ar of 35.45.
Cl-35 → 77.5%
Cl-37 → 22.5%
Hint: What do you know about the total natural abundance of both isotopes?
IB Exam Tip: It is only possible to solve this type of problem when there are only 2 isotopes.
Practice 3:
Practice 3:
Lithium has 2 naturally-occurring isotopes—Li-6 and Li-7. Calculate the exact natural abundance of each isotope, given that lithium has an Ar of 6.94.
6.94 = (6 x a) + (7 x (100 - a)) / 100
694 = 6a + 7(100 - a)
694 = 6a + 700 - 7a
694 = -a + 700
694 - 700 = -a
-6 = -a
6 = a
Natural abundance of Li-6 → 6
Natural abundance of Li-7 → 100 - 6 = 94
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