University of Virginia
Physics Department

## Atomic Mass

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.4

Objectives

Students will

• investigate atomic mass and the manner in which it's measured;
• calculate atomic mass of elements given the numbers of electrons, protons, and neutrons;
• determine the number of neutrons in an element given the atomic mass and the number of electrons.

Motivation for Learning

Driving Question

How big is an atom? How much does it weigh? How do we "weigh" it? How can we tell how much "stuff" is in it?

Background Information

This activity is designed as an introduction to atomic mass. (We will assume students are familiar with atomic number and understand that it comes from the number of protons in an atom.) It is difficult for students to understand how we know the mass of atoms when they are so small. It may also be difficult for them to that when we determine the mass number for an element, we only count the numbers of protons and neutrons (since the mass of an electron is so much smaller than that of a proton or neutron). It is important to understand what particles really affect the mass in order to later understand what is different with different isotopes of elements.

### Student Activity

To print out the Student Copy only, click here.

Materials

• Marbles (fairly heavy) of two different colors (to represent protons and neutrons)
• Small (very light) beads (to represent electrons)
(These are just suggestions for materials - any suitable substitutions will work. You just need to make sure that the protons and neutrons have approximately the same mass and that the electrons have a much smaller mass.)
• Balance
• Container that can be sealed and covered (some options are a baby food jar, racquetball container, tennis ball container, etc.)
-- you want to make the containers "atoms" by placing the correct number of particles inside of the container (ex. If you want to make carbon, you would place 6 "protons", 6 "electrons", and 6 "neutrons" in a container and seal it up.)
-- you also may want to write the mass of the empty container on the outside so that the students won't have to determine the mass of it.

Procedure

1. Divide students into groups (3-4 students would be a good size). Explain that they will be using items to represent the different particles of an atom since the real particles are too small to handle.
2. Give each group a "proton"(marble), a "neutron" (different color marble), and an "electron" (bead). (You either want the particle attached to an index card with the name or make a key on the board so that students will remember which item represents which particle.) Discuss the fact that these particles are the building blocks for atoms. These are the three things that are put together to make up an atom. Ask questions like "Which particle is the heaviest?" "Which particle is the lightest?" "Where does most of the mass come from in an atom?" You want to lead them to the fact that atomic mass mostly comes from the protons and the neutrons.
3. Have the students mass each particle and record that information in the data table. (Depending upon the precision of your balance and the weight of the objects you're using, it may be necessary to take the mass of more than one particle in order to obtain a reading on the balance. The accuracy of your numbers will also improve if you follow this procedure. For example, you may want to take the mass of 50 "electrons" and divide that number by 50 to calculate the mass of one of those "electrons.") Do their data agree with the observations they made earlier about the heaviest and the lightest particles?
4. Give each group an "atom." (You want them to be able to open this container so that later they can count the particles inside.) Have them take the mass of the "atom." They will need to subtract the mass of the empty container so that they only get the mass of the particles inside.
5. Have them open up the atom and count the number of each particle and record that in the data table. Multiply the number of each particle by the mass of one of those particles and record that answer in the data table. Add all of the particles' masses to find a total mass of the atom and compare that to the measured mass.
6. Have the students in the different groups compare the number of protons to the number of electrons. (Since you will be using neutral atoms, these should all be equal; therefore, the students will be able to determine the number of protons given the number of electrons and vice versa.)

Data Sheet

To print out the Data Sheet only, click here.

Data Table:

 Mass of one particle "protons" "neutrons" "electrons"

Atom 1:

 Mass of atom + container = ______________ g Mass of empty container = ______________ g Mass of atom = ______________ g
 "protons" "neutrons" "electrons" -------------- # of particles -------------- Total mass from particle Total Mass

Atom 2:

 Mass of atom + container = ______________ g Mass of empty container = ______________ g Mass of atom = ______________ g
 "protons" "neutrons" "electrons" -------------- # of particles -------------- Total mass from particle Total Mass

Extensions

Have students try to come up with a way to determine the number of jellybeans in a jar.

Students with Special Needs

Each student should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

Assessment

1. Give each group a new "atom". (Make this one sealed so they can't count the number of particles, but have to determine it based on the mass.) Tell them the number of electrons inside and have them determine the number of protons and neutrons.
2. Give students a table of information including the number of electrons, protons, neutrons and the atomic mass of the atom. Have blanks in the table and have them fill in those blanks. For example:

 Element # of protons # of neutrons #of electrons Atomic mass Potassium 19 20 Sulfur 16 32 Lead 125 82