This lesson is an introduction to periodic trends. It has students use the Science and Engineering Practice (SEP): Analyzing and interpreting data. Students work in groups to graph and analyze three periodic trends: atomic radius, ionization energy and electronegativity. These are the traditional names for these the trends; however, I've been informed that one of the goal of NGSS is to avoid focusing on terminology, and instead emphasize the concepts. In this case the key concept is the ability of atoms to gain or lose electrons based on electrostatic attraction. Therefore I have chosen to not use the traditional names of the trends because I want students to understand the concept of electrostatic attraction, not the trends defined by definitions.
If this seems counterintuitive, this lesson can easily be modified by inserting the traditional trend names for what I use:
This lesson is aligned with NGSS PS1-2- Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties. The periodic table orders elements horizontally by the number of protons in the atom’s nucleus and places those with similar chemical properties in columns. The repeating patterns of this table reflect patterns of outer electron states.
This lesson continues to engage students in the Crosscutting Concept Patterns. This is seen as students continue to build on the concept that electrostatic attraction is a driving force in chemistry.
Class starts with students answering the question in their journals, "how does chlorine attract an electron to obtain a full valence shell?"
This is followed by each student sharing their thoughts with the person next to them. After a few minutes of individual thinking, and paired sharing, we have a group discussion about the question. Typical student thoughts are:
These answers are all correct, but ultimately do not answer the question of how chlorine gains an octet. The conversation has to be guided towards electrostatic attraction because high school students still don’t fully understand this concept. Because electrostatic attraction is an underlying concept critical to understanding all periodic trends it is important to address this idea at the beginning of the lesson.
As groups start to mention the attraction between protons and electrons, as a result of my questions or from their own thinking about oppositely charged particles, this is a good time to bring up trends and introduce the central activity for the lesson.
After the class conversation has been directed towards electrostatic attraction, I ask the class to name some trends they've seen in the periodic table.
Trends usually mentioned are metals, nonmetals, increasing atomic number, atomic mass and number of valence electrons are the same in groups.
As we are talking about these trends I pass out a sheet of Graph Paper and copy of the trends they will be graphing. After all student have their papers, I assign them a number 1-3 to represent each of the following trends: (1) electron removal energy, (2) atomic size and (3) electrostatic attraction.
I then instruct them they will be sharing their graph with two other people during the next period to complete an assignment that summarizes each trend. It is important that everyone’s graph is as accurate as possible.
Graphing takes a good portion of the class because I emphasize that graphs should be as accurate as possible since they will be shared with other students to complete the activity in the next lesson. As they are working on their graphs I walk around helping them space the numbers.
I find it important to show them that graphs should have the trends on the y-axis and elements on the x-axis. I then show that the 18 element should be graphed by divided the the number of spaces on the x-axis by the number of elements, spaced accordingly after rounding up. And the trend should be spaced according to the difference between the highest number and the lowest number, divide by the number of spaces on the y-axis. This can be seen in the post lesson reflection and a copy of each trend graphed.
The purpose of having students graphing trends first is to help them develop a deeper understanding by seeing it first, instead of just hearing about it. It also provides them with a crucial component of the NGSS Practice of Analyzing and Interpreting Data by having to create a graph from data and interpret other classmates; instead of just reading and interpreting a graph that has been created already. The process of creating the graph helps them understand the data more authentically because they created it.
As a fun assessment I have the students that have a Twitter account tweet me the answer to a trend based on what they graphed.
At the end of the class I put my Twitter (specifically made for my classes) on the board with the 3 questions. I instruct students to answer the question that relates to what they graphed via a Twitter message. I don’t expect all students to be assessed this way, but I think it is fun and effective way to differentiate instruction for the students that like technology.
As an alternative assessment, students can answer the questions on a Post-it note that is at their table and place it on the wall on their way out of the classroom.
I found this to a be a successful lesson because many of the students worked on graphing skills that will be used throughout, and were able to develop an basic understanding of periodic trends which traditionally is a difficult concept. I was able to assess their understanding after many of the students were able to answer the 3 formative assessments. These were relatively easy trend questions, but for a first day of learning them they were plenty challenging. As the unit progresses they will have to broaden the understanding by understand the trend across the periodic table as well.