Isotopes Lab

**PURPOSE:**

This exercise will demonstrate the relative abundance of isotopes and explain how the weighted atomic mass of the elements is determined. Pennies from 1982 are similar to a mixture of isotopes in that they all look the same, but their masses tend to group around two values rather than one. This difference is due to a change in the minting process from solid copper to copper with a zinc core. Therefore, holding two pennies, one that is solid copper and one that is copper with a zinc core is similar to holding isotopes of an element in your hand.

**MATERIALS:**

balance; 20 pre-1982 pennies; 20 post-1982 pennies; 20 1982 pennies.

**PROCEDURE:**

- Work in pairs and obtain 20 pre-1982 pennies. Record their average weight to the nearest 1/100th of a gram. This represents the lighter isotope we will call "X".

- Now obtain 20 post-1982 pennies and record their average weight to the nearest 1/100th of a gram. This represents the heavier isotope that we will call "Y" (example 3.24g).

- Next obtain 20 1982 pennies and record their average weight to the nearest 1/100th of a gram. Here we have mixed isotopes with an average weight of (3.09g)

- Determine the relative abundance of each isotope using the following formula:

Use the following formula to determine the percent of the mixed group that are pre-1982 type isotopes and post-1982 type isotopes.

avg. wt. of 1982 coins = [avg. wt. of "Y" + avg. wt. of "X" / 2 ]

For the avg. wt. "X" we can substitute (1 - Y) because "X" can be determined by subtracting "Y" from 100%.

Example: 3.09 = [3.24 + (1 - 3.24 X) / 2 ]

**CONCLUSION:**

- Why is the average weight used instead of the weight of one coin?

- How can you tell pre-1982 from post-1982 coins without looking at the date or weighing them?

- Do the following problems and find the atomic mass using significant figures.