Download isotopes and average atomic mass

Chemistry Lab
Atomic Structure
Name __________________________6/06
Date ___________ Period ______
ISOTOPES AND AVERAGE ATOMIC MASS
INTRODUCTION
The atomic mass of an element is the sum of all the masses of the sub-atomic
particles which comprise the atom. The mass in grams of these particles (protons,
neutrons, electrons, et al) are exceptionally small. The mass of the proton is 1.67 X
10-23 grams. The neutron is slightly larger and the electron is much smaller. For
any given atom, the number of protons and electrons can not vary. However, the
number of neutrons may vary from atom to atom of the same element. This causes
the mass of atoms of the same element to vary. Atoms of the same element with
different atomic masses are called ISOTOPES.
To obtain the best value of the mass of an element, we would need to consider
the various masses of the atoms of that element and the percentage of all atoms of
that element with those masses. We would need to find the "weighted" average of
the masses.
Consider the diagram below:
14
12
12
14
12
12
12
12
12
12
12
12
14
12
The average mass be for the sample of atoms below is not thirteen due to many more
atoms which weigh twelve:
In this lab you will determine the "atomic mass" of the hypothetical element
Beanium (Bn).
PROCEDURE
1. Obtain a sample of Beanium atoms.
2. Determine the total number of atoms in your sample.
3. Count the number of atoms of each isotope.
4. Calculate the percent abundance of each isotope by dividing the number of atoms
of each isotope by the total number of atoms.
5. Calculate the mass of each isotope by massing 10 atoms on the electronic balance
and dividing by 10 (you are therefore determining the mass of ONE typical atom of
the isotope.)
6. Calculate the average atomic mass of Beanium (Bn) by multiplying the percent
abundance of each isotope times that isotope’s atomic mass for each isotope and
adding all of the products.
DATA
1. Total number of "atoms"
2. Number of isotope 1 “atoms”
Number of isotope 2 “atoms”
Number of isotope 3 “atoms”
3. Mass one (1) atom of isotope 1
Mass of one (1) atom of isotope 2
Mass of one (1) atom of isotope 3
4. Mass of all Beanium atoms (actual)
CALCULATIONS: Show all work!!! (Since all numbers are “exact values” calc. All % to +/- .1%)
1. % isotope 1
% isotope 2
% isotope 3
2.
Average atomic mass of Beanium
= (% isotope 1) (mass isotope 1) + (% isotope 2) (mass isotope 2) +
(% isotope 3) (mass isotope 3)
3.
Actual average mass (mass of all
Beanium atoms on electronic balance
4.
% error = average atomic mass – actual mass x 100%
average atomic mass
QUESTIONS:
1. What is an isotope?
2. Are C-14 and N-14 isotopes? Explain your reasoning.
3. How do the different beans represent isotopes?
4. In a given sample element X-25 is found in 65.0% abundance, element X-27
in 25.0% abundance and element X-29 in 10% abundance. Is the weighted
average mass closest to X-25, X-27 or X-29. Justify your answer.
5. Calculate the average atomic mass of the substances below:
% Abundance
symbol
Mass (amu)
75.53
Cl-35
34.969
24.47
Cl-37
36.966