I have found that this lab is not very useful for students of high-school age. They find it tedious and obvious. Nevertheless, I will leave it posted here in case someone else thinks otherwise or would like to use it with younger students. This notice will not print. Try this activity instead: Lab: Atomic Mass and Average Atomic Mass

Atomic Mass
and Beans


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.

black.beans (17K)


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.

Categories.Atoms.Elements.Isotopes (7K)

All matter is made of atoms.

Some atoms are metallic, for example, iron (Fe).

Iron has the atomic number 26 (Z = 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.


Mathematical Toolkit
p+ + n0 = A
protons + neutrons = atomic mass
(Z + n0 = A)
To find the mass of one bean:
count out 25 beans,
find the mass,
divide by 25

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  1. Find the mass of a plastic cup. You will subtract this mass from all others to find the mass of what is inside the cup.
  2. Find the mass of one black bean. Use 25 beans. Place them in a cup, find the mass, subract the mass of the cup, and divide the result by 25.
  3. Find the mass of one white bean. Follow the same procedure to find the mass as used with the black bean.
  4. The mass of one white bean and one black bean should be close. Find the average. This is the scaled up mass of 1 atomic mass unit (amu). Remember, protons and neutrons each have a mass of approximately 1 amu.
  5. Build the atoms below in the following way:
    1. Fill in the correct symbol, number of p+, number of n0, and number of nucleons (A). Use your periodic table!
    2. Count out the number of protons (black beans) into a cup.
    3. Find the mass of the protons alone.
    4. Count out the number of neutrons (white beans) into another cup.
    5. Find the mass of the neutrons alone.
    6. Combine the protons and neutrons in one cup to build an atom.
    7. Find the mass of the atom.
    8. Divide the mass of the atom by the average mass of a bean.
  6. 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.
  7. Repeat the previous step for each of the five atoms you teacher has put together.
    Note for Teachers: use nitrogen-15, neon-20, sodium-23, chlorine-35, and potassium-39
  8. Identify the atoms clearly when you record and report these data.


  Mass (g)
black bean
white bean
average bean
Name Symbol Z
No. of p+
Mass of
p+ (g)
A - Z
No. of n0
Mass of
n0 (g)
# of nucleons
Total Mass
p+ + n0 (g)
Total Mass ÷
Avg. Bean
carbon-12 126C              

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Answer the following questions using complete sentences. Unclear or incomplete answers will receive no credit.
  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? What is it similar to? 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? Write the data you collect here, making your own table. (Use a ruler).
Last updated: Nov 12, 2006 Home  |   Home