Ray O - Isotopes

Title: Element 127: Beanium, 8th Grade Physical Science


Principle Investigated: Relative Atomic Mass


California Science Content Standards Grade 8:

            7. The organization of the periodic table is based on the properties of the elements and reflects the structure of atoms. As a basis for understanding this concept:

            b. Students know each element has a specific number of protons in the nucleus (the atomic number) and each isotope of the element has a different but specific number of neutrons in the nucleus.


            To help students understand how the naturally occurring isotopes of an element affect the relative atomic mass of a sample of that element.


Prior Knowledge:

            Students should know that each atom has a nucleus consisting of protons and neutrons, and this nucleus is surrounded by moving electrons that are a great (relative) distance away, meaning the atom is mostly empty space.

            Students should know that almost all the mass of an atom is in the nucleus, and that both protons and neutrons have a mass of approximately 1 u (unified atomic mass unit).

            Students should know that atoms of each element contain nuclei that always consist of the same number of protons.

            Students should know that the number of neutrons in atoms of an element can vary (but even when the number of neutrons in nuclei varies in atoms of that element, the atoms are still the same element as long as the proton number is the same), and that atoms of an element with different numbers of neutrons are called isotopes.

            Students should know the term average, and know how to calculate the average of a set of numbers.




Three varieties of beans

Balance or digital scale



Activity Procedure:


(1) Select one of the numbered containers, each holding many Beanium atoms.  Each type of bean represents an isotope (version) of the element Beanium.  Be sure to record the number of the container you chose.

(2) Sort the Beanium atoms into groups based on the types of beans (pinto, navy, kidney). Record the total number of atoms (beans) in your sample, as well as the number of each type of bean.

(3) By following the instructions in the Analysis section, find the mass of each isotope of Beanium.  Using this information, determine the relative atomic mass of Beanium.

After the students have gathered all data and returned equipment, guide the class through the analysis.

Additional questions for discussion:


(1) What is the basic atomic difference between isotopes of the same element, other than atomic mass?

Answer: Different isotopes of an element have different numbers of neutrons, which causes the difference in mass. The different numbers of neutrons can also affect the stability of the nucleus.

(2) Why are the atomic masses on the Periodic Table not listed in whole numbers?

Answer: Samples of elements typically contain more than one isotope of the element, meaning that the mass number is actually an average mass of the varying isotopes present in the sample. It's unlikely this average will happen to be a whole number. Also, neutrons are slightly more massive than protons, and this can affect the the final mass figure.
(3) Aluminum has a relative atomic mass of 27 (rounded off), but the isotopes of aluminum have mass numbers of 24, 25, 26, 28, 29, and 30. Why are they all considered to be aluminum atoms?
Answer: They all have 13 protons in the nucleus. Therefore they are all aluminum, even though they each have different numbers of neutrons and, consequently, different masses.
Beanium Activity Worksheet: