University of Virginia
Physics Department

Atomic Number and Mass

A Physical Science Activity

2003 Virginia SOLs

 

Objectives

Students will

 

Background Information

The atomic number of an element indicates the number of protons in the nucleus of an atom of that element. For example, an atom of carbon has 6 protons in its nucleus so its atomic number is 6. No other element has atoms with 6 protons in their nuclei. The atomic mass of an element indicates the total number of protons and neutrons in the nucleus of an atom of that element. A proton and a neutron have almost the same mass. Scientists use atomic mass units (amu) to report the mass of atoms. Protons and neutrons each have a mass of 1 amu. On the periodic table, the atomic mass for carbon is listed as 12.011. If you round this figure down to 12, then it indicates that carbon atoms have 12 protons and neutrons in their nuclei. Since we know that carbon has 6 protons from its atomic number, then we can calculate the number of neutrons by subtracting the atomic number from the atomic mass (atomic mass - atomic number). In this case, carbon has 6 neutrons in its nucleus. So why is the atomic mass of carbon listed as 12.011 on the periodic table? Most elements consist of two or more isotopes. Isotopes are atoms of the same element, but with different atomic masses. Carbon has three isotopes: carbon-12, carbon-13, and carbon-14. All of these isotopes of carbon have atoms that contain 6 protons in their nuclei. The difference between these isotopes is the number of neutrons in their nuclei. Carbon-12 atoms have 6 neutrons, carbon-13 atoms have 7 neutrons, and carbon-14 atoms have 8 neutrons. Any sample of carbon will contain all three isotopes. This is why the atomic mass for carbon is 12.011. Which isotope of carbon do you think is the most common?

 

Student Activity

Materials

Procedure

Part I (Atomic Number and Atomic Mass)

  1. On a chalkboard, assign one of the colors of your marbles to represent protons and the other color to represent neutrons.
  2. Explain to your students that the number of protons in the nucleus determines the identity of an atom. This number is called the atomic number.
  3. Place one of the marbles that represents a proton into the flask. Ask students what kind of atom this represents and what its atomic number and atomic mass are. (Hydrogen; 1; 1).
  4. Place another "proton marble" into the flask. Again ask students what kind of atom this represents and what its atomic number is (helium; 2). Also ask them if there is anything missing (the neutrons). At this point, ask students how many neutrons helium should have (Helium has two neutrons). Go ahead and place two "neutron marbles" into the flask. Ask students what helium's atomic mass is. (Helium has two protons and two neutrons so its atomic mass is 4).
  5. Continue for the first 10 elements or further if you have enough marbles.

 

Part II (Isotopes)

  1. Place 6 "proton marbles" and 6 "neutron marbles" in a flask. Have students identify the element. (Carbon, specifically carbon-12).
  2. Add another "neutron marble" to the flask. Again ask students to identify the element. (It still is carbon, specifically carbon-13).
  3. Again add another "neutron marble" to the flask. One last time, ask students to identify the element. (It still has 6 protons, so it is still carbon, specifically carbon-14). This is a good time to discuss isotopes.
  4. Work through some other common examples of isotopes.
Hydrogen-1, Hydrogen-2 (Deuterium), and Hydrogen-3 (Tritium)
Helium-3 and Helium-4
Nitrogen-14 and Nitrogen-15
Oxygen-16, Oxygen-17, and Oxygen-18

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