6.2 Isotopes and Average Atomic Mass

Topic 6.2
Average Atomic Mass

The mass of an element on the periodic table is not the mass # or atomic mass but is rather the average atomic mass.

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Average Atomic Mass

The average atomic mass is a weighted average which includes the mass of each isotope and the percent abundance of each isotope.

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Average Atomic Mass

To say that an isotope is 10% abundant is to say that 10% of ALL atoms of that element consist of isotopes with that particular mass.

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Percent Abundance

Percent Abundances for all isotopes must add up to 100%

The mass of all isotopes must be accounted for

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Percent Abundance

Math Response

Assume an element has three naturally occurring isotopes. X-32 is 25% abundant, x-33 is 20% abundant, therefore the last isotope, X-34 is _____ % abundant. (type in 2 numbers and no unit)

Type answer here

Correct Answer: 55

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Rad

Calculating Average Atomic Mass

​Average Atomic Mass = (mass Isotope x abundance) + (mass of isotope x abundance) + .......

​For this equation you want the "abundance" to be the decimal form of the %abundance therefore divide the % abundance by 100 prior to putting the value into the equation

Math Response

Use the following data to calculate the average atomic mass of the following hypothetical element (round to the 10th place and include the units amu and no spaces)

X-25.2 12.5% abundant

X-26.8 80.5% abundant

X-27.9 7.00% abundant

Type answer here

Correct Answer: 26.7amu

Deg^°

Rad

Math Response

Determine the % abundance for isotope X-48.9 (answer to the 10th place and no unit)

X-44.0 33.0% abundant

X-46.0 25.0 % abundant

X-48.9 ___ % abundant

Type answer here

Correct Answer: 42.0

Deg^°

Rad

Math Response

Calculate the average atomic mass for the following hypothetical element (answer to the 10th place and label with amu; no spaces)

X-44.0 33.0% abundant

X-46.0 25.0 % abundant

X-48.9 ___ abundant

Type answer here

Correct Answer: 46.6amu

Deg^°

Rad

Multiple Choice

An element has 3 naturally occurring isotopes with the following percent abundances:

X-12 50% abundant

X-13 25% abundant

X-14 25% abundant

Which isotope would most greatly affect the average atomic mass and why?

• X-14 because it has a larger mass

• X-12 because it is most abundant

• unable to determine answer

• X-13 because it is in the middle of of 12 and 14

Multiple Choice

The average atomic mass of an element is found to be 10.812amu

Which of the following isotopes is most likely to be the most abundant?

• B-11 because it is closest to the average atomic mass

• B-10 because it is closest to the average atomic mass

• B-12- because it is the largest mass

• B-9 because it is the smallest mass