An atom of lithium using the Azer atomic model.

Atomic Number and Mass Number with the APH Azer Model

This simple activity allows students to display their knowledge of basic concepts related to the atom using the APH model.

Students use the APH atom model to both become familiar with the concept of atomic number and mass number and to display (model) their understanding of these concepts.  These concepts are much clearer and more concrete for students who are blind or visually impaired when taught with a model.  I find the APH model to be an excellent resource.  The atom model is part of the Azer Periodic Table Set and can also be purchased separately 



After initial instruction on the structure of the atom and the use of the AZER atom model, set up a different atom for each member of the class using the APH atom model.  See picture above

If you are not familiar with this model, it is available using quota funds through APH.  Many Region Service Center here in Texas already own a set or more.  I usually start with the “early” atoms from the Periodic Table; Hydrogen, Helium, Lithium, Beryllium, and Boron so that the atomic number and mass number will be easy to count.


  1. Pass out an atom to each student and tell the students which type of atom (element) the student has.  
  2. As you instruct the students on atomic number and mass number, allow the students to determine each using the model.  In my class I read the section on these concepts to the class as each student determined atomic number and mass number for their atom.  Alternatively, you could provide students with a written definition of each term in the appropriate reading medium at the beginning of class.
    • This activity serves the added purpose of helping students to become more familiar with the AZER atom model as they learn related concepts.
  3. Each student will orally (or in written form) communicate the atomic number and mass number of his/her atom.  Because each student has a different atom, answering aloud is ok.


Define related terms:



Have students switch atoms with another student and repeat the activity.  Have students check each others calculations and discuss.   


Once students are comfortable with the concepts of atomic mass and mass number, have students calculate the following:

  1. Calculate the number of neutrons in an atom given atomic mass (# of protons) and the mass number (# of protons and neutrons)        
  2. Calculate the atomic number (# of protons) given the atomic mass (# of protons and neutrons) and number of neutrons.  The formulas for these calculations are below, but have the students work to figure out the formulas themselves.    

They will accomplish this task using only subtraction.  

Mass number – atomic number (number of protons) = number of neutrons 

Mass number – number of neutrons = atomic number (# of protons)

NGSS Standards

High School: Structure and Properties of Matter
PS1.A: Structure and Properties of Matter

Each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons. (HS-PS1-1) The periodic table orders elements horizontally by the number of protons in the atom’s nucleus and places those with similar chemical properties in columns. The repeating patterns of this table reflect patterns of outer electron states. (HS-PS1-1)

By Laura Hospitál

collage of atomic number

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