Atoms vs Ions
Lesson 8 of 13
Objective: SWBAT apply the octet rule to determine if an element will lose or gain electrons, and how many, when becoming an ion.
This lesson gets to the heart of HS-PS1-1: Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms. Now we get to predicting elemental behavior based on the valence structure. We continue to rely heavily on pictorial representations, based in Science and Engineering Practice 2- Developing and Using Models.
The focus on ion formation is what atoms do to become stable, so this lesson is the first to access the Cross Cutting Concept of Stability and Change. In particular, we are setting up the high school level that states students will understand "Much of science deals with constructing explanations of how things change and how they remain stable." A major focus in teacher talk today is in how elements get to a stable electron arrangement, like the noble gases.
I got this worksheet from a colleague, who found it online. It is nearly EXACTLY what I wanted to make for this lesson. Unfortunately, I have not been able to find it myself to give proper credit for its creation. If you know the origin of this work, please let me know so I can provide them the proper credit.
Whole Class Introduction
This video shows how we begin the lesson on the differences between atoms and ions. This is done via the document camera after all students have a paper and have put their names on the paper.
Paired Work Time
After modeling for the whole class, I give students time to work with a partner to become more comfortable with the process. Students may choose a partner from a different table and move to the lab space, or work with a partner at their current table.
I remind students of our Bohr-Rutherford models from a couple days ago as a way to help them make the neutral atom drawing faster.
I point out the reminders on the top of the page regarding how cations and anions are formed. I encourage them to write down that cations are formed by neutral atoms with 1-3 valence electrons, and anions are formed by elements with 5-7 valence electrons.
While students are working, I circulate the room to provide reinforcement, and answer questions. Whenever possible, I refer students back to the hints at the top of the page, their Bohr-Rutherford diagrams, and their Valence Electron notes. I am trying to foster some independence in my students and break their need to always ask me questions.
When we are close to the end of partner time, I give a two minute warning before we return to our seats at the front tables and check in.
Whole Class Check-In
When we stop the action, I ask all students to find Phosphorus on page 2. I ask students to walk us through the process, asking for different information from each table. One table tells us the periodic table information, another the numbers of each particles, and a third explains how to complete the Bohr model.
Now I pause and ask "How does this Phosphorus atom obey the octet rule and fill its valence shell?" Some students will respond without thinking "Give up its electrons" due to having just completed four examples who all give up their electrons.
I then prompt "How do we know when elements will give up or gain electrons?" and someone will check the front side and point out "It has 5, so it's going to gain electrons" If the student doesn't state it gains 3 electrons, I ask "If it has 5, but needs 8, how many will it gain?".
When we make our ion diagram, I encourage students to use a different symbol for the gained electrons, either an "X" or a checkmark. We then fill in the particle information for the ion, and write the correct symbol.
I caution students to pay extra attention to how many valence electrons they have before deciding if elements will gain or lose electrons going forward. I then instruct them to work independently now, but allowing them to check answers with their table mates after completing each element. Students are on the honor system to check answers instead of copying, and I reinforce this while walking the room. When students have different answers, I ask them to talk out the process to see who made the error, and will model it for them. "Would this element gain or lose electrons? How many? So what's the difference between protons and electrons now? So what is the new charge?"
Individual Work Time
While students work, I continue to circulate the room and check their work. Students may start to notice patterns in the periods, particularly in how the ions are getting to the same total number of electrons. I encourage these discoveries, and assist students who might be having difficulty in completing the diagrams or ion information on the charts.
When students finish, I ask them to turn in their work on the front table, or to turn it in at the end of the period regardless of completion so I can check it.
Students had some difficulty in switching between metals and non-metals. Approximately 75% showed mastery and being ready to move on, but the remaining had difficulties. The most common difficulty is in either ALWAYS adding or removing electrons regardless of the rules or original amounts. The second most common difficulty is in students who are still misinterpreting the information from the periodic table, using the mass number as the numbers of each particle. The final area of trouble is in creating the Bohr model of the neutral atom. When this is done incorrectly, students can not ascertain the correct changes to become an ion.
For next year, I plan on altering this activity. I would provide the Bohr model and particle information for the period 2 neutral atoms. In the beginning, I would refer to how we knew that information already, and only have students do that part independently on elements from period 3. This would focus students on the core content of ion formation, and also help speed up the process a bit. It would also provide additional practice and review at using the periodic table and creating Bohr models, but make that practice faster after students saw multiple correct representations.