Isotope Lab Activity - In Search of Atomic Mass

Name: _________________________   Class: ____     Date: ___________

The extremely small size of atoms makes it impossible to count them or determine their individual atomic masses using direct means. An instrument called a mass spectrometer allows such determinations.

In this activity you will investigate the various isotopes of some "make believe" elements.

The Mass Number of an atom is the sum of the number or protons and the number of neutrons in the nucleus of an atom. In reality, not all atoms of an element have the same number of neutrons. Atoms of the same element that have different numbers of neutrons are called isotopes. Because of this variance in nature, scientists define mass number as the average atomic mass of an element (See page 278 -281 in your textbook).

Problem

How can you calculate the average mass of a mixture of objects?

Objectives

v      Using colored beads, make a model to represent the average atomic mass calculations of isotopes.

v      Calculate the average mass of given samples.

Materials

Various colored plastic beads

Activity #1

In this activity you will calculate the average mass of a "make believe" element known as "eimerium". Eimerium (Ei) has two different isotopes. One isotope contains 286 protons and 110 neutrons while the second isotope of this element consists of 286 protons and 111 neutrons. Isotope one can be written as Ei-396 and the second as Ei-397.

Obtain a sample of this element. Count the number of "turquoise" beads. These represent the Ei-396 isotope. Record this number in the data table. Count the number of "tan" beads. These represent the Ei-397 isotope. Record this number in the data table. Determine the average atomic mass for this element.

Table 1

Activity 2

Determine the abundance of Isotopes found in "nature" from the above activity. Assume that in "nature each Isotope appears as often as the teal and peach beads found in the activity. Calculate a percent for each isotope. (Hint: Remember, percents always = 100%

Abundance of Isotope Ei-396 =  ______________                Abundance of Isotope Ei-397 = _________________

Activity 3

Many elements have more than two Isotopes. This activity has three Isotopes. Determine the average atomic mass for the element Crosbinium. Determine the percent of abundance for this element. Complete the data table. The make believe element Crosbinium has 301 protons and either 115, 116, or 120 neutrons.

Element: Crosbinium                Isotope Cb-416 (Red)                Isotope Cb-417 (Green)                Isotope Cb-421 (Tan)

Table 2

Activity 4

Obtain your sample labeled "Jacksonian" This sample is to be handled differently. You will take "scientific samples" of this substance and determine the amounts of Isotopes found in nature, their abundance and the average atomic mass of your "sample".

Scientists often have to sample substances to obtain data about them. In this activity you will take "samples" of the substance and average all the samples together to determine its "make up". Jacksonian atoms contain 314 protons and anywhere from 125 to 140 neutrons. When sampling use the following scale to determine the mass of each atom sampled. NOTE: don't forget to add the protons to the neutrons to get the atomic mass of each atom.

Dark blue = 125 neutrons                turquoise = 126 neutrons                pink = 127 neutrons                red = 130 neutrons

Cream = 133 neutrons                green = 135 neutrons                Clear = 140 neutrons

Based on the above information, the atomic mass of each Isotope would be as follows:

1.        Open the container for your sample. Carefully remove 50 pieces (atoms) from the sample. Do not "look" into the sample, rather randomly pick out your fifty atoms. Record the data you receive in the table provided. This is your first sample.

2.        Return all "atoms" to your sample container. Take a second sample in the same manner as the first. Remove 50 "atoms" from the container and record the data in the space provided.

3.        Continue doing steps 1 and 2 again until you have taken five samples.

4.        Determine the abundance of each isotope found and the average atomic mass of this substance.

Table 3

Questions

1.        The element Boron is a common element. It contains two common isotopes boron-10 and boron-11. In a sample of boron atoms you found that 4 atoms were boron-11 and 1 atom was boron-10. Calculate the actual atomic mass for boron. (Show your work).

What is the abundance (in %) of each of Boron's Isotopes?

2.        How are the isotopes of an element alike and how are they different?

3.        Using your textbook (page 280) explain how isotopes are used in medicine.

4.        Explain how to find the average atomic mass of any element?

5.        What information can be obtained from the modern periodic table?

Sample Data answers from Fall 2000

This activity was written and produced by Dave Crosby 11/14/00