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

Isotope	Number of Atoms	Mass of Atoms	Total Mass of Sample
Isotope Ei-396 (Turquoise)
Isotope Ei-397 (Tan)
		Combined Mass
		Average Atomic Mass

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

Isotope	Number of Atoms	Mass of Atoms	Total Mass of Sample	Abundance (%)
Cb-416 (Red)
Cb-417 (Green)
Cb-421 (Tan)
		Combined Mass
		Average Atomic Mass

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:

Isotope (Color)	Protons	Neutrons	Total Atomic Mass of Isotope
Dark Blue	314	125	439
Turquoise	314	126	440
Pink	314	127	441
Red	314	130	444
Cream	314	133	447
Green	314	135	449
Clear	314	140	454

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

Sample	Dk Blue Jk - 439	Turquoise Jk - 440	Pink Jk - 441	Red Jk - 444	Cream Jk - 447	Green Jk - 449	Clear Jk - 454
1
2
3
4
5
Total Sample (A)
Mass of Atom (B)
Total Mass of Sample (A x B)
Total Mass 0f Sample (All Isotopes	___________________________________ amu
Average Atomic Mass	___________________________________ amu
Relative Abundance of Each Isotope (%)

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?