Exercise 1Isotopes, Mass Number, and Atomic Mass In this exercise, you will create diagrams of common isotopes and relate atomic number and mass…

Exercise 1 Isotopes, Mass Number, and Atomic Mass

In this exercise, you will create diagrams of common isotopes and relate atomic number and mass number to subatomic particle numbers. You will also calculate atomic mass from isotope mass and abundance data.

ProcedurePart 1: Isotope Diagrams

  1. Figure 3 illustrates a nucleus in which each letter “n” represents a neutron and each letter “p” represents a proton.

Figure 3. Nucleus of Isotope A.

  1. Count the number of protons and record in Data Table 1.
  2. Count the number of neutrons and record in Data Table 1.
  3. Determine the atomic number and record in Data Table 1.

Note: Reference the background for the definition of atomic number.

  1. Determine the mass number and record in Data Table 1.

Note: Reference the background for the definition of mass number.

  1. Determine the isotope name:
  • Reference the atomic number and periodic table to determine the element name.
  • Write the isotope name as “element name – mass number”. For example, the element with atomic number = 1 and mass number = 3 would be written as Hydrogen-3.
  1. Record the isotope name in Data Table 1.
  2. Create a diagram of Isotope B, similar to Figure 3, adding the appropriate number of “n” and “p” for an atom with the mass number of 13.

Note: The nucleus and symbols can be drawn by hand or illustrated with computer software.

  1. Add your name and date to the drawing.
  2. Take a photo of the drawing and upload the image into Photo 1.
  3. Complete the Isotope B column in Data Table 1.
  4. Repeat steps 8-11 for Isotope C with an atomic number of 14, uploading the image into Photo 2.

Part 2: Atomic Mass

  1. Calculate the estimated atomic mass of each element in Data Table 2 using the formula below.

Atomic Mass (amu)




Mass A


Percent A)


(Mass B


Percent B)








Atomic Mass (amu)=[(Mass A×Percent A)+(Mass B×Percent B) + ….100%

  1. Solve the equations on a sheet of paper that includes your name and date.
  2. Take a photo of your work and upload the image into Photo 3.
  3. Record the calculated masses for each element in Data Table 2.

Exercise 1 – Questions

1. What are isotopes? How do isotopes of a given element differ? How are isotopes of a given element similar?

2. Explain the difference between mass number and atomic mass.

3. How many protons are in the nucleus of a nitrogen-15 isotope? How many neutrons?

4. How do your calculated atomic masses of oxygen and sulfur compare to the atomic masses listed in the period table: oxygen = 15.999 amu, sulfur = 32.065 amu? If your answers differed, explain where the error may have been incurred.

5. Copper has only two naturally occurring isotopes, copper-63 and copper-65. Use the atomic mass of copper from the periodic table to determine which must be the most abundant.

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