Isotopes

Objectives:

5.2 Identify the advantages and disadvantages of using isotopes in industry, medical science, basic research, and in the environment.

5.3 Analyze the irregularities in the progression of the atomic masses of the elements in the periodic table.

436

Irregularities Of Mass In The Periodic Table

Elements do not have their atomic masses as whole numbers.

The atomic mass of an element is the weighted average of all the different isotopes of that element

This is also known as Relative Mass.

Calculating Relative Mass

To calculate relative mass do the following:

You need to know the relative abundance of each isotope (expressed as a percent %)and the atomic mass of each isotope.

Use the formula below to solve for relative mass.

Relative Mass =

(amu * % abundance) + (amu * % abundance)

100

Using the information given below about Lithium, find its relative mass.

amu % AbundanceLi-6 6u 7.42%Li-7 7u 92.58%

Relative Mass = (6u * 7.42) + (7 * 92.58) 100

Mass = (amu * % abundance) + (amu * % abundance)

100

Mass = 6.93u

Using the information given below about oxygen, find its relative mass.

amu % Abundance

O-16 16u 99.762%O-17 17u 0.038%O-18 18u 0.200%

= (16u * 99.762) + (17 * 0.038) + (18 * 0.200) 100

Relative Mass = 16.008

The atomic mass (relative mass) of carbon is 12.01113719 amu. The chart below shows data on the two isotopes of carbon. What is the atomic mass of the second isotope of carbon?

Isotope Atomic Mass % Abundance

C-12 12.00000 98.89%

C-? ? ?

Find % abundance of second isotope…. 100% - 98.89 = ?

= 1.11%

1.11%

Use the formula to solve for the atomic mass (working backwards)

12.01113719 = (12u * 98.89) + (? * 1.11%) 100

Relative Mass = (amu * %abun.) + (amu * %abun.) 100

Atomic mass = 13.00335

13.00335

Silicon (Si) has 3 naturally occurring isotopes. One isotope has 14 neutrons and a % abundance of 92.2, another isotope has 15 neutrons and a % abundance of 4.70, and finally the last isotope has 16 neutrons and a % abundance of 3.09.

What is the approximate average atomic mass of Silicon?

TRICKY!

Get the atomic mass for each isotope….

% AbundanceSi-28 99.762%Si-29 0.038%Si-30 0.200%

NOTE: The question only gave the neutrons….so you add them to the protons!

% AbundanceSi-28 92.2%Si-29 4.70%Si-30 3.09%

Plug information into formula and solve

= (28u * 92.2) + (29u * 4.70) + (18u * 3.09) 100

Relative Mass =27.7352u

Calculating relative mass:

Relative Mass =

amu x % abundance + amu x % abundance

100

Atomic mass

Activities and Resources

Worksheet # 2 Student Study Guide, Module I p.

I-50 Other Activities:

ANSTO (Australian Nuclear Science and Technology Organization)

http://www.ansto.gov.au/ari/brochures_misc/rad1.html

References

Student Study Guide – Physical Science 416/436 - MEQ

Science Quest – Grenier, Daigle, Rheaume – 1998, Cheneliere

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