This lesson is based on California's Middle School Integrated Model of NGSS.
MS-PS1-1: Develop models to describe the atomic composition os simple molecules and extended structures.
DCI: PS1.A: Structures and Properties of Matter - Substances are made from various types of atoms, which combine with one another in various ways.
Science and Engineering Practices: (2) Developing and Using Models
CCC: (2) Scale, Proportion, and Quantity. (5) Energy and Matter
Understanding the interior of the atom allows students to comprehend the larger picture of atomic interaction and molecular development. (MS-PS1-1) (PS1.A) (CCC 5). This lesson allow students to interact with a model that correctly depicts the scale of an atom, if the proton was roughly the size of a bowling ball and the electron was the size of a BB (S&E Prac 2). Very few middle school textbooks mention the true scale of an atom, which in this case places the electron almost eleven miles away (CCC 2).
My goal with this lesson is to reduce the chance of a science misconception and set my students up for success in high school, college, and/or graduate school. The internal structure of an atom can be complex due its abstract nature. To that end I support my students as much as possible with models such as these. (unPD - grabble with complex tasks, using scientific models).
Building this model requires seven large styrofoam spheres, three small wooden dowels and three BBs. I painted this model to highlight the particles.
Electrons in action
To show the correct scale, I created a Google Map of the atom. I dropped a pin on my classroom to designate the nucleus, then placed two electrons at eleven miles away to represent the first electron shell. The scale isn't exactly right, but it gives the kids a good sense of the atom. I placed eight electrons twenty-two miles away to represent the second electron shell.
Using Google Maps
I found this tutorial very helpful in creating my own map. Creating Maps Using Custom Maps for Google Maps
I hang the nucleus model in the center of my classroom and ask the kids, "What is this?"
Some kids respond that it is an atom, I wait until someone identifies it as a nucleus. We then talk about which nucleus it represents. The green spheres represent protons, so eventually someone counts and tells me this is a lithium (Li) nucleus. I ask how they arrived at that answer. The responses typically include that they counted the protons and looked up the atomic number on a periodic table. I make sure to reenforce the concept that the number of protons is the atomic number.
I then ask the class what the atomic mass of this nucleus is. By counting all the particles (three green protons and four orange neutrons) they arrive at an atomic mass of seven amus (atomic mass units - every number needs a label). We talk about the atomic mass on the periodic table is an average which can be rounded to seven. I make sure to identify it as an isotope, since it has one more neutron tun it should have (perfect atoms have the same number of protons, neutrons, and electrons - very few atoms remain perfect).
I then hold up the three electrons and demonstrate how two electrons orbit in the first shell and the third electron orbits, by itself, in the second shell. I remind them that we learned that in nature a single electron in an outer shell is typically discarded. Counting the protons (+3) and the electrons (-2) we arrive at an electrical charge of +1. I remind the class that any atom with an electrical charge is referred to as an ion.
I ask the kids where I should place the electron at this scale to correctly represent the atom. By this time most of the class has figured out that the electron should be far away, but very few kids understand how far away it should really be.
I switch gears and use the projected image of their school on the overhead screen. Most kids identify their school correctly and like seeing it as a satellite image.
For a bit of fun I tell the kids that this is a live view of the school with only two seconds of lag time. If someone would volunteer (pick someone who doesn't mind being made fun of, I typical choose two) to run around outside waving their arms we could see that on the screen. We all get a big laugh when this kids outside realize that I am displaying a two year old Google Map photo.
At this point I zoom away from the center pin (nucleus) using the +/- button on the map. Each step I allow the kids to familiarize themselves with the map and pick out landmarks. At each step I ask them where the electron is. When the first shell finally appears I show some familiar landmarks nearby to give the class a sense of scale.
We finish by zooming out to the second shell and of the understand of how far the electron should be at the scale we are using.
I created a custom worksheet of my area so my students could draw a scale model of an atom. To create it I took a screen shot of Google Maps with my location (center of map /location of nucleus) marked.
Use this worksheet as a template to make your own custom worksheet. To create the scale distance mark I measured the scale distance (10 miles) on the Google Map (lower right corner) with a 'Post-it' note and recreated that measurement on the worksheet.
Some students may interpret this model as the nucleus being spread out within the first electron shell. They may see the first shell as the boundary holding the nucleus together, instead of the nucleus being a very compact structure.