This lesson is based on California's Middle School Integrated Model of NGSS.
MS-PS1-1: Develop models to describe the atomic composition of simple molecules and extended structures.
DCI: PS1.A: Structure and Properties of Matter - Substances are made from different types of atoms, which combine with one another in various ways.
S&E Practices: (2) Developing and Using Models
CCC: (5) Energy and Matter
This demonstration model is designed to give students an opportunity to see the inside of an atom and experience the atomic composition of several atoms (MS-PS1-1). Models are an important learning tool, in that they provide an simpler representation of the phenomenon being studied. (Practice 2 Developing and Using Models - Develop a model to predict and/or describe phenomena.)
As with all models, there are limitations that must be addressed. Specifically, this model does not adequately show that electrons orbit in three dimensions, the electron is depicted as being too large in comparison to the proton, and finally the electron shells are too close to the nucleus. It is important to discuss these limitation with students in order to dispel misconceptions that may arise in high school or college level chemistry courses. (SP2 - Evaluate limitations of a model for a proposed object or tool.)
If students are to comprehend the larger picture of molecules and compounds they must understand how an atom is depicted, along with subatomic particles of protons, neutrons, and electrons (PS1.A). The larger subject of matter overlaps with other subjects to provide a comprehensive picture of the nature of matter form the atomic level to the macro level (CCC 5).
In developing this activity, I am trying to present a model of an atom that can be manipulated by myself or by students in order to satisfy their line of questioning. I also want students to use correct scientific vocabulary (nucleus, electron shell, proton, neutron, electron) to describe the structure of the atom.
This model of an atom was originally built by Meg Deppe - Science Coach: Apple Valley Unified School District.
Directions for assembly
1. Screw 1 small eyelet into the bottom side of the large 36" hula-hoop (blue).
2. Screw 2 small eyelets into the top and bottom side of the small 26" hula-hoop (green).
3. Attach a small length of chain to the large 36" hula-hoop. This will allow the model to hang from the ceiling (used a large paperclip to create an attachment point).
4. Attach a small length of chain from the eyelets of the large 36" hula-hoop to the top of the small 26" hula-hoop. Position the small 26" hula-hoop in the exact center of the large 36" hula-hoop.
5. Attach a small length of chain from the eyelet of the small 26" hula-hoop to the large 14" metal ring. Position the large 14" metal ring in the exact center of the small 26" hula-hoop.
6. Attach a small length of chain from the large 14" ring to the hamster ball. Position the hamster ball in the exact center of the large 14" ring.
7. Place small lengths of clear tape around the 36" and 26" hulls-hoops and large 14" ring to correspond to the locations of electrons in the shells.
I hang the model in the middle of the classroom and pass out a copy of Atom Model Demonstration Worksheet to each student. The students use three colored pencils to do this activity (not required to actually do this activity - I think students learn more when they work with colors). One colored pencil represents protons, another represents neutrons, and the third colored pencil represents electrons.
I start out explaining that the atom has a nucleus (point to the hamster ball), which contains the protons (hold up one large colored fuzzy ball) and the neutrons (hold up a different large colored fuzzy ball). I make sure that each student has a copy of the periodic table in front of them.
Tip: Assign colors on the board representing each particle. i.e. green-protons, yellow-neutron, and red-electron. The actual colors you use is not important, there is no standard. Whatever you choose make sure to stick with it.
I then explain that the electron (hold up the small fuzzy ball) reside in shells around the nucleus (point to the rings/hula-hoops). I tell them that some very large atoms can have up to seven shells, but I only have physical room to depict three). The first shell (point to large 14" ring) can only hold 2 electrons. The second shell (point to 26" hula-hoop) can hold 8 electrons. The third ring can hold an additional 8 electrons (point to the 36" hula-hoop). Additional shells (not depicted) can hold large amounts of electrons, partly due to their large size.
I also explain that in past grades (5th grade in CA) they learned a basic atomic model and that atoms have an equal number of protons, neutrons, and electron. While this is technically correct, nature doesn't listen. I call these types of atoms "perfect atoms" and tell the kids that very few atoms remain perfect.
The first atom I build with the kids is carbon (atomic #6), which is done for them on the Atom Model Demonstration Worksheet. I add 6 protons (count out loud in a dramatic fashion) and 6 neutrons to the nucleus, explaining that the proton count represents the atomic number and the total contents of the nucleus (hamster ball) represents the atomic mass. I then add 2 electrons to the first shell and make sure to point out that the first shell is now full and ask the students what I should do next. Most of the kids understand that I need to put the remaining 4 electrons in the second shell. I have the kids color in example #1 on the Atom Model Demonstration Worksheet using the pre-assigned colors.
I then build lithium (atomic #3). I again count out three protons and three neutrons, but stop to make sure the kids know that the contents of the nucleus (hamster ball) represents the atomic mass (protons + neutrons). According to the Periodic Table the atomic mass of Lithium is 6.941 amu (I round to 7 for the sake of simplicity). I then show the kids that I need to add an extra neutron in order to get the atomic mass right (becomes an isotope). I then add the electrons, again stopping and asking what I should do after the first shell is full with 2 electrons. I then introduce the Octet Rule (I don't mention the name of the rule, only the concepts - that can wait until high school).
The Octet rule says that atoms prefer to be as much like a Nobel Gas as possible. (I point out that atoms don't really have "wants and desires", since they are not living things, but personifying atoms depicts this rule pretty well). Since the second shell only has 1 electron, lithium will try to be like helium (atomic #2) as much as possible and give its outside electron (known as a valance electron) to the universe. This sets up lithium as having 3 protons and 2 electrons for an overall electrical charge of +1 (becomes an ion). After I have built the atom the students will draw what they see in example #2, color and label the particles, and identify the atom.
The next atom I build is sodium (atomic #11). I again count out 11 protons, but to make the atomic mass of 23 (22.990 amu) I need to add 12 neutrons (isotope). As with lithium I then add 11 electron, but the last electron (valance electron) is lost to the universe (negative ion). The kids then document this atom on their worksheet in example #3.
The final atom I build is chlorine (atomic #17). I add 17 protons and 18 neutron (isotope) to get an atomic mass of 35 amu (35.453 amu). I then add 17 electrons and explain that chlorine wants to be like argon so it will add an electron (valance electron) to have an overall +1 charge (positive ion). As before, the kids document what they have seen for their worksheet in example #4.
I find it very helpful to introduce the concept of a valance electron early as it explains why chemistry is able to follow the rules as it does. I build on the concept of valance electrons in my lesson Sodium (Na) Reaction Demonstration - it involves explosions!
As I am building each atom (carbon, lithium, sodium, chlorine) the students are documenting what they are observing on their Atom Model Demonstration Worksheet. Each example must use three pre-assigned colors (green-protons, yellow-nuetron, and red-electron). On their own they must identify the atomic symbol, atomic number, and atomic mass.
TIP: In the last student example #2, the student incorrectly labeled Lithium as having 3 electrons. As you filter around the room check for understanding in regards to correct particle (proton, neutron, electron) count.
I have found three puzzles on Science Spot that allow the kids to work with vocabulary in a non-threatening way.