I like having students look at data and then explain data based on what they know about periodic trends. This activity uses the data we all use for radii and ionization energies and asks students to look just a little bit deeper.
I have gone back and forth between using this as an in class activity (my current practice) and using some of these questions on exams.
A student will be able to
- use experimental data to identify periodic trends; and
- provide complete explanations for the periodic trends observed in experimental data using Z*, n and electron configuration arguments.
Currently, I use this as an in class activity in my lower division class; the first two pages take an entire class period (with interspersed lecture). I use this as review when I teach the second semester (which is separated in time from the lower division class). The seniors finish this activity in the class period.
I begin class by asking students to answer things based on what they remember from general chemistry, then to look at the data and use the explanations we've discussed (size - based on shells of electrons, Z* and electron configurations where things aren't always changing in the same way) to explain these periodic trends. (I've only started the learning cycle bit this year. I've found that my students have persistent misconceptions from general chemistry / high school. I'm hoping that by letting them acknowledge these ideas and discuss where they fall down may help their understanding and retention of the material.)
After the brief instructruction and introduction to the activity, I have students work in groups of three. (They have assigned groups for the whole semester.) My two peer leaders and I go around and answer questions when they come up. If I hear the same question a few times, I'll bring the class together to talk about the question. (I use Doceri so that's pretty easy to do from anywhere in the room.)
I try to leave 5 minutes to wrap up at the end of class.
I award students credit for participation, not correctness. (These in class activities not only help the students work through the material, but they also encourage them to be there for an 8 AM class 3 days a week.) If students participate in 75% of the in class activities, they get full credit for this 5% part of their grade. (I only require 75% because I don't have to keep track of student absences for university activities, illness, misset alarms, etc.)
I read over these every night, make notes on things that I think are significant problems, and return them the following class period. If I see anything that is a consistent problem, I'll discuss it at the beginning of the following class period.
Students have no trouble stating the trends, but they have more problems explaining the arguments. I find that they tend to make incomplete or incorrect arguments arguments. For example,
- the radius is smaller as you go across a period because Z increases
- Z* increases because sheilding increases
- the size of the atom increases because the radius is larger
There is also a tendency to go back to the "it's extra stable because of the half-filled or completely filled subshell" that they learn in general chemistry.
While the students are working, I find it really valuable to sit down and discuss how to make a more complete argument about periodic trends.