- University of Virginia
- Physics Department
Atomic Number and
A Physical Science Activity
2003 Virginia SOLs
- identify the first ten elements based on their atomic number and atomic
mass using a simple model;
- understand the concept of isotopes and identify some common examples using
a simple model.
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?
- 250 ml flask
- 20-30 marbles (10-15 each of two different colors) (or colored candy such
as M&M's or Skittles)
- Periodic Table of Elements
Part I (Atomic Number and Atomic Mass)
- On a chalkboard, assign one of the colors of your marbles to
represent protons and the other color to represent neutrons.
- 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.
- 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).
- 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).
- Continue for the first 10 elements or further if you have
Part II (Isotopes)
- Place 6 "proton marbles" and 6 "neutron marbles" in a flask.
Have students identify the element. (Carbon, specifically
- Add another "neutron marble" to the flask. Again ask students
to identify the element. (It still is carbon, specifically
- 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.
- 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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