Download In actual laboratories, isotopes in a sample can be

Academic ChemistryH
Chapter 4
Isotopic Mass and Atomic Mass
Name: _______________________
Date: ______________ Pd: ______
Background: Elements are composed of atoms. These atoms are composed of protons, neutrons, and electrons. Essentially, the
protons and neutrons that are present in the nucleus determine the mass of an atom. The mass of the electron is so small
that chemists generally ignore it in most applications. The number of protons present in the nucleus identifies the atom.
For example, atoms with 17 protons are chlorine atoms. However, not all chlorine atoms have the same number of
neutrons. Atoms of the same element (i.e., with same number of protons) that have a different number of neutrons are
called isotopes. One isotope of chlorine has 18 neutrons and a different isotope has 20 neutrons. These atoms would be
considered isotopes of chlorine. The masses of isotopes vary due to their different numbers of neutrons, but their chemical
properties are the same. It is important to note that isotopes are not abnormal atoms – most elements are made up of two
or more isotopes. Also, the abundance of each isotope within an element is consistent. This activity will allow you to
model the process of determining a weighted average, and for solving problems related to isotopes and atomic masses.
Materials: balance, sample of the element “Beanium” composed of a variety of isotopes, beakers of various sizes to hold the
different isotopes
Procedure: Read each statement carefully. Information is provided about completing all calculations related to atomic mass.
1) Observe the sample of the element Beanium. Observe each “isotope”. How many isotopes are present? What differences
do you observe? What difference is the most useful one for our model to represent isotopes of a real element?
2) In actual laboratories, isotopes in a sample can be separated from each other by using a device called a
mass spectrometer. When gaseous atoms of an element are passed through a magnetic field, they will separate into
different ‘rays’ according to their masses. We will sort the beans by “sight” into different groups. Record the number of
each type of bean, as well as the total number of atoms of each type. Use this data to calculate the percentage abundance
of each isotope. (Remember: percent expresses the ratio of a part to the whole.)
3) The mass spectrometer separates the isotopes by using differences in their relative masses. You will determine the mass of
each individual isotope by finding the total mass of each group of beans, and dividing by the number of beans in that
group. (Unlike our beans, real atoms of the same isotope all have the same mass, so we will need to assume that this
“averaged” mass represents all the beans in that group.) Applying this to atoms, this mass is called the isotopic mass for
each isotope. Record your results.
4) The atomic mass of an element on the periodic table is a weighted average of the naturally occurring isotopes. This is
calculated by the formula:
Atomic Mass =[ (Isotopic Mass)1 x (%Abundance)1 ] +[ (Isotopic Mass)2 x (%Abundance)2 ] + etc.
The number of [terms] in the formula depends on the number of isotopes present in the sample. Use this formula to
determine the atomic mass of the element “beanium” in your sample. (Use the decimal form of each percentage.)
5) Compare your average mass of beanium with other groups from class. Why might there be differences? Would you expect
to find differences if samples of real elements were studied?
Data:
Isotope
Number of atoms present
Total Mass
(mass of each group)
Isotopic Mass
(mass of 1 atom)
Percent Abundance of this
isotope in the whole sample
1
2
3
4
Total
6)
Show your calculation here for the atomic mass of Beanium:
XXXXX
100%
Analysis and Related Questions: Complete the problems neatly on loose-leaf paper. Show all work. You can expect to
see problems like these on the chapter 4 assessment. These problems are due at the beginning of the next class period.
1)
Chlorine exists naturally as chlorine-35 (isotopic mass = 34.969 amu, percent abundance =74.95%) and chlorine-37
(isotopic mass = 36.966 amu, percent abundance = 25.05%). Use this information to calculate the atomic mass of the
element chlorine.
2)
The following data is for mercury. Use the data to calculate the atomic mass of mercury to the hundredth’s place.
Remember to use decimal equivalents for the percentages.
Isotope
Isotopic Mass (amu)
Abundance
Hg-196
195.97
0.14%
Hg-198
197.97
10.04%
Hg-199
198.97
16.83%
Hg-200
199.97
23.12%
Hg-201
200.97
13.23%
Hg-202
201.97
29.79%
Hg-204
203.97
6.85%
3)
Neon consists of three isotopes with masses of 19.99 amu, 20.99 amu, and 21.99 amu. The three isotopes are present in
nature to the extent of 90.92%, 0.25% and 8.83% respectively. Calculate the atomic mass of neon.
4)
Magnesium has three isotopes: Mg (23.985 amu) = 78.99%, Mg (24.986 amu) = 10.00%, plus a third isotope. Look up the
atomic mass of magnesium on a periodic table and determine the isotopic mass of the third isotope.
5)
Gallium consists of two naturally occurring isotopes with masses of 68.926 and 70.926 amu respectively.
a.
How many protons and neutrons are in the nucleus of each isotope?
b.
What is the mass number of each of these isotopes? (Remember – mass numbers are whole number sums.)
c.
Look up the atomic mass of gallium. How does it compare to the isotopic masses above? Does it look like the
element is made up of exactly 50% of each isotope? ____ Explain how you know.
6) Calculate the percent abundances of each isotope of gallium. (Hint: This problem needs a different approach from the first
three. You are trying to find out the percents by knowing the isotopic masses and the final average. If you let the percent of
the first isotope be “x”, what must be the other percent, in terms of x? _____Why can you assume this?_______________
Remember that percentages must be expressed in decimal form for computation.)
7) Silver consists of two isotopes, Ag-107 (mass = 106.905 amu) and Ag-109 (mass = 108.905 amu). Look up the atomic
mass of silver and calculate the percent abundance of each isotope.
8) Nitrogen consists of two isotopes, N-14 and N-15. The mass of each isotope is 14.0031 amu and 15.001 amu. The atomic
mass of nitrogen is 14.00674. Calculate the abundance of each.