Lab Team #____ Date________________ Period _________
Names: __________________ _________________ _________________ _________________
Atomic Mass and Beans
Objective: In
this lab you will get some help visualizing the make-up of atomic nuclei. You will use
differently colored beans to represent protons and neutrons. Both of these particles reside in the atomic
nucleus and can be called by one name: nucleons.
Background: Atoms are placed in categories called elements. Atoms belong to an
element if they have the right number of protons in their nucleus. The number of
protons is called the atomic number and elements each have their own atomic
number. Within each element there are varieties of atoms called isotopes. Isotopes of
an element all have the same number of protons but different numbers of neutrons.
All matter is made of atoms. Iron has the atomic number 26. This means all atoms of iron have 26
protons. Not all atoms of iron are exactly the same. All of them have 26 protons but some have more or
fewer neutrons than others. These are the isotopes of iron: 5426Fe,5626Fe,5726Fe,5826Fe.
Materials
 beans
of same size, 2 different colors; cups; lab balance; lab notebook; pen; calculator
Mathematical Toolkit
mass number = p+ + n0
atomic mass = weighted average of all isotopes of an element.
Procedure: Develop and record your own procedures (when requested on data table) to meet the following objectives.
1. Find the average mass of one pink bean (proton.)
2. Find the average mass of one red bean (neutron.)
3. Use this information to determine the value of the average bean. This represents 1 “atomic mass
unit” (a.m.u.) in our bean nuclei.
4. Build the atoms on the table below in the following way:
o Fill in the correct symbol, number of p+, number of n0, and number of nucleons (A). Use
your periodic table. Record on data table in columns #2, 3, 5, 7, respectively.
o Count out the # of protons (pink beans) into a cup.
o Find the mass of the protons alone & record on data table in column #4.
o Count out the number of neutrons (red beans) into another cup.
o Find the mass of the neutrons alone & record in column # 6.
o If time permits, combine the protons and neutrons in one cup to build an atom.
o Find the mass of the atom. (If time is short, just add the mass of p+ and mass of n0.)
Record in column #8 of data table.
o Divide the mass of the atom by the avg mass of a bean (1 “a.m.u.”)&record in column #9.
5. Obtain one of the atoms your teacher has prepared for you. Identify these atoms and collect the
same data about them that you collected for the atoms you built yourself.
6. Repeat the previous step for each of the five atoms you teacher has put together.
7. Identify the atoms clearly when you record and report these data.
Data
Average Mass
(g)
Pink bean
(proton)
red bean
Explain procedures & show calculations used
to determine mass.
(neutron)
average
bean
(1 “a.m.u.”)
1.Name
carbon-12
2.Symb
ol
3.Atomic #
(No. of p+)
4. Mass of
p+ (g)
____beans
5. No. of
n0
6. Mass of
n0 (g)
Red beans
7. mass #
(# of
nucleons)
8. Total
Mass
p+ + n0 (g)
9. Total Mass
÷
Avg. Bean
10. Atomic
Mass of
element
12
6C
carbon-13
carbon-14
oxygen-16
oxygen-17
oxygen-18
magnesium-24
magnesium-25
magnesium-26
iron-54
iron-56
iron-57
Unknown A:
_______
Look up
values
Unknown B:
________
Unknown C:
________
Unknown D:
_________
Unknown E:
__________
Analysis & Conclusion
Answer the following questions using complete sentences.
1. What makes the isotopes of an element different from each other? There are two ways to express
the answer to this question: use both.
2. What do all atoms of a certain element have in common?
3. What is the meaning of the last column in the data table? Justify your answer.
4. The actual mass of a proton is 1.673 x 10-24 g. What was the actual mass of a ‘proton’ in this lab?
Why do scientists use atomic mass units instead of grams?
5. The actual mass of a neutron is 1.675 × 10-24 g. Does it still make sense to say that the mass of
one neutron equals the mass of one proton at 1 amu? Why? What in this lab is similar to this
difference in the real masses of protons and neutrons?
6. What is the identity of the five unknown atoms? Record the data you collect.
7. Based on the known atomic mass, which isotope of your 5 unknowns seems to be the most
common?
Isotope
Relative Abundance in Nature
carbon-12
98.9
carbon-13
1.1
carbon-14
trace
oxygen-16
99.8
oxygen-17
0.0004
oxygen-18
0.002
magnesium-24
79
magnesium-25
10
magnesium-26
11
iron-54
5.8
iron-56
91.7
iron-57
2.2
Unknown A:
nitrogen-15
Unknown B:
neon-20
Unknown C:
sodium-23
Unknown D: chlorine-35
Unknown E: potassium-39