Graphing Trends Day 2
Lesson 11 of 12
Objective: SWBAT summarize the patterns in reactivity across periods and down groups/families of the main group elements at the macroscopic level using the periodic table.
This lesson is a follow up to Introduction to Periodic Trends. My students will use the Science and Engineering Practices (SEP): Analyzing and interpreting data and modeling. In this class period they use the graphs made in the previous lesson in addition to diagraming Bohr’s models of atoms to identify trends in atomic radius, electrostatic attraction and energy to remove an electron
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 as students continue to build on the concept that electrostatic attraction is a driving force in chemistry.
At the start of class I have a 1, 2 and 3 on opposite sides of the room. I use this to place students in groups according to which graph they worked on from the previous day. The previous day students were assigned a graph that either illustrated (1) atomic radius, (2) electrostatic attraction or (3) energy to remove an electron. Once students are in their groups, I arrange them into groups that have a 1, 2 and 3 (one of each graph).
After groups have been arranged I send each group to separate tables and pass out the day’s assignment. If there are students that did not complete the graph, I put them in groups and allow them to take pictures of my Graphs and use them to complete the assignment. I also encourage all students to take pictures of their group's graphs so they can study them for the test.
Students that completed the graph will receive credit for the completing the previous days assignment. Most students receive credit because they were given 20 minutes in the previous class to complete their graphs. While I am checking in their graphs, I am looking for accuracy so that the assignment is done correctly. If there is a problem with any of the groups graph, I have them use a copy of my graph.
After passing out the assignment I show the class what their graphs should look like and provide them with a brief description of each of the trends. For graph 1 I tell them that as you move across the periodic table from left to right the energy to remove an electron increases and that energy decreases as you move down a group.
While showing graph 2 I explain that as elements change from right to left across the periodic table atomic radius decreases going from top to bottom and radius increase from top to bottom.
Overall I don’t want to explain too much about the trends because this is the purpose of the day’s assignment, but I do explain atomic radius in detail because they have a difficult time understanding the trend moving across a period.
As I am showing them graph 2 I will draw a Bohr model of Li, O and Ne on the board. In the middle (nucleus) of each diagram I will put the number of protons and then draw the electron shells as shown in the video.
I then show them graph 3 and explain that as elements change across the periodic table from left to right the ability of an atom to attract an electron (electrostatic attraction) increase and decreases as moving down.
At this point I again relate to the previous diagram for atomic radius and ask which is going to have a stronger pull on an electron, 3 protons or 8 protons? As a review I explain that Ar will not attract an electron because it has an octet already.
I then explain that as you down the periodic table the attraction decreases because (1) the atom is getting bigger and the electrons are further away and (2) there are more electrons shielding the nucleus due to electron repulsion.
Shielding is a new concept, but students understand it because they realize that likes repel.
After showing the graphs I illustrate the trends on the period table and have them do the same on their periodic table.
After completing the explanation portion of the lesson I instruct students how to do the activity. This activity has them elaborate on the trends by modeling a trend by comparing two Bohr’s models and explaining why one atom satisfies a trend. This helps them see how size and electrostatic attraction are related. It also helps them visualize that it is easier for an electron to be removed as they get further away from the nucleus.
To guide students in the right direction I model 1a. (Which atom is larger, Carbon or Oxygen?) by drawing a Bohr’s model of each and explaining that carbon is bigger because there are less protons pulling on the valence shell.
This proves to be a good example to diagram for them because students still struggle with atomic radius moving across a period. See reflection.
The remainder of the period will be used to complete this assignment (students work)
As students are working on the elaborate portion of the lesson, I am walking around checking for understanding. I don't formally evaluate in this lesson because I want to leave as much time as possible to complete the assignment.
Students typically struggle with the idea that as an atom gains more valence electrons, the atom will decrease in size. This concept is very difficult for students to grasp.
The evaluation for this part of the lesson will occur at the beginning of the next class period in the form of quiz