In this lesson students learn how to determine the number of different types of subatomic particles in atoms, what an isotope is, and how to calculate a weighted average.
There are several resources that I utilize in this activity.
In order to engage students in the lesson I begin by having students revisit the model of the atom that they stared in the previous lesson.
In this section I introduce students to the idea of how we can determine the number of subatomic particles in an atom through using atomic number and atomic mass.
I use the Bohr model of the atom to show students how to draw models reflecting the number of protons, neutrons, and electrons. At this point I do not have students worry about where they put the electrons. I tell them, "to put two electrons in the first energy level (circle), eight in the next, and up to eighteen in the third energy level, and that we will discuss more about where to put electrons in the next class."
I present information using the first six slides of the attached PowerPoint and students fill in information on the first page of their Graphic Organizer. Here is a copy of one student's filled in notes.
Some students get confused when doing examples when they see the atomic mass. I make sure to stress to students to round to the nearest whole number before they subtract atomic number from atomic mass.
In this next section of the lesson students are given time to explore three isotopes of Hydrogen to help them determine the definition of an isotope. Students do this using the top half of the second page of their notes graphic organizer.
My goal is that students are able to determine that all isotopes have the same number of protons, same number of electrons, and same atomic number (questions number 2 and 5). Additionally, they are to determine that isotopes have a differing number of neutrons and atomic mass (questions 3 and 4). Through this analysis their definition of an isotope should be that they are atoms that have the same atomic number (or number of protons/electrons), but different atomic mass (or number of neutrons).
Over the years I found that students struggle with the concept of isotopes so this way of helping them figure out the definition on their own helps students to better remember the concept.
As students complete the analysis I review answers on a blank copy of the graphic organizer and call on groups to help with the answers to the questions. As students answer I fill in the answers on the top part of the graphic organizer as can be seen in this example.
Here is a Video showing how this works in my classroom in terms of leading students in the process of discovering the definition of isotopes.
In this section students have time to synthesize what they have learned about atoms and isotopes through completing two group activities:
In this section of the lesson I help students deepen their understanding of isotopes through understanding the idea of weighted averages. I lead students using slides 12 - 17 on the Power Point and they fill in the second half and third page of the notes graphic organizer.
I scaffold this lesson by having students first think about normal straight averages through using examples of test results. I then go through examples of how to calculate weighted averages using the same test results, but with weights applied.
This Video shows how I am able to scaffold new chemistry content using the idea of weighted averages.
After this, I lead students through examples dealing with Chemistry. I help students a lot with the first problem, and then give them time to work through the other problems on their own and walk around to help out. As students complete each problem I go over answers on the board.
Here is an example of student work with answers to the questions.
For the last part of class students have time to begin their homework. Any work they do not complete in class they will finish at home and bring in the next day when I will stamp and review. This is the answer key to the homework.
On the homework most students feel comfortable with calculating protons, neutrons, and electrons.
The major area where students struggle is with calculating average atomic mass. I make sure to review the answers by going through examples on the board and stress that they just need to follow the steps that we went over in class.
Although weighted averages is an important concept, it is not one that I end up testing students on. I find that it is a "nice to know" concept, but not one that I stress that all students understand.