Bohr's Model and Valence Electrons

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Objective

SWBAT explain our current understanding of the atom by citing evidence from experiments they conduct

Big Idea

Rutherford's experiment illustrates the nature of science and reveals the structure of the atom.

Introduction

This lesson addresses the NGSS 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".  The goal of this lesson is to continue building on students' prior knowledge from the previous 4 lessons in unit 1; taking their knowledge of subatomic parts, their charges and the relative size of the atom and model the location of electrons in shells.  This is aligned with the NGSS Disciplinary Core Idea (DCI) PS1.A (Structure and Properties of Matter): “each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons”. 

The students explore the atom using the NGSS Practices of Developing and Using Models.  Modeling the atom is a key practice when studying the atom because they are too small to observe.  Today students will model Bohr’s model of the atom to determine how many electrons fit in the first 3 orbitals of an element. This is essential because it will lead to a better understanding of the periodic table and bonding.

The Cross Cutting Concept (CCC) that is illustrated throughout the lesson is Patterns.  This is illustrated as students see a key pattern on the periodic table that relates to the number of electrons in an atom. This idea of patterns is an underlying theme and students will not be summatively evaluated on this part of the lesson.

Warm-up/Engage

10 minutes

As class starts students  answer the warm-up that addresses the lesson from the previous day in their science journals:  "What did you learn about the atom from Rutherford's gold foil experiment?” and “What are some issues that were not explained in his experiment?”  

This prompt has students think about the process of what went into discovering the atom and what things were left out of Rutherford’s experiment based on the model experiment they performed with blocks and marbles.  Students tend to understand that the marble (which was positive) ricocheted off the blocks, but forget that like charges repel.

Several desired response to the warm-up are:

  • Rutherford discovered a dense, positively charged nucleus;
  • there is a lot of space between the nucleus and electrons;
  • the marble was positive therefore it was reflected back; and
  • the experiment does not explain exactly where electrons are and if they are just floating around.

 

I do a demonstration to illustrate this -- I take the largest magnets I can find and show them what occurs when two positive sides of a magnet are put together.  This demonstration gives a good visual of the process and can be done before they start the warm-up, or during the discussion after it is completed.  When I show the demo depends on how much students remember from the Rutherford experiment.  I usually do it during the discussion (after completion of the warm-up) because I want students to recall what they did the previous day.  I also encourage students to talk to one another before they complete the warm-up.

During the discussion portion of the warm-up the idea the nucleus was positive is mentioned and subsequently followed by the notion that electrons are present.  As the conversation is developing I ask students, “so are the electrons randomly floating around”?  This leads to conversation that electrons are found in shells or orbitals, and because of this they contain energy and are attracted to the nucleus.

I then ask, “How many shells do you think are present in an atom?"  

This solicits some guesses, but usually aren't close to explaining how they are arranged.

I Follow this by asking, “do you think rules are involved in the placement of electrons in the shell(s), if so what might they be”?  

Students also say yes which create enough curiosity where Bohr can start being discussed.

I don't go any further in my explanation about electron placement because HS-PS1-1 only requires that students know electrons contain energy and have attractive forces necessary for bonding, Students do not have to know the specific electron configuration depicted in the Quantum Mechanic model.

Explain/Guided Practice

10 minutes

The discussion from the warm-up leads directly into notes from the Atomic Theory ppt on Bohr and his atomic model (slides 6-8) of the atom and the quantum mechanical model (slide 9).  The main focus of this lesson is the Bohr’s model, but I think it is important students realize there is a more accurate version of the atomic model that will not be studied. 

We stop at the Bohr model because the NGSS standard (PS1.A) does not require that students learn electron configuration. They only need to know the concept of valence electrons, which can be illustrated using the Bohr’s model.   

I take a few minutes to discuss Bohr (slide 6) so that students understand what he did in relation to Rutherford.  I quickly get into his model (slide 7-8) and discuss the rules of how electron are arranged in different shells. 

It is important to know I will discuss this model using elements up to Argon, otherwise the 2,8,8 rule will not apply.   The purpose of learning this model is see a pattern develop in the arrangement of valence electrons as atoms move across the periodic table. 

Towards the end of the notes I will hand out the Making Bohr Models worksheet.  I have the class read the instructions and then I demonstrate how to make the example from the worksheet on the board. 

By this time in the unit students should be getting fairly comfortable with reading the elements on the periodic table, but this activity will require them to practice writing the atomic number, mass number, number of protons, neutrons and electrons. 

Explore

20 minutes

The materials that will be needed for this activity are note cards, three round objects to make the rings (a compass works well too) and markers or colored pencils (optional).  I tell students the day before to bring their own note cards.  This works well enough that you will only need to provide some students with note cards.  As an alternative, students can also make note card size pieces of paper and cut them out; however, this takes more time and will lead to students having homework.

As students are making their cards I walk around checking to see if they are putting all the information on the cards that was discussed in the instructions (see worksheet).  The process takes students a little time to get started, but after a few cards are made the process goes quicker. 

Thirty minutes is usually enough time to finish the cards, but the conclusion questions will take longer and should be assigned for homework.  This is explained further in the elaborate section of the lesson and is why there is 0 minutes of time for the elaborate section.

The purpose of this assignment is to show students that the number of valence electrons can change as elements change (PS1.A), but also that patterns of electron arrangement can be determined. 

This knowledge is paramount for the next couple of lessons in this unit and the following unit about the periodic table and bonding.  This activity also touches on the idea that atoms gain and loss electrons to fill its valence shell.  This is a secondary concept that students will need to learn later, but I believe that it is good to expose them to this early so that it will not be a new concept in the bonding unit.

Elaborate/Homework

After students have finished their cards they will arrange them so they represent the periodic table arrangement, and answer the conclusion and synthesis questions on the worksheet using their arrangement.

As mentioned in the previous section, this will probably be homework for many of students.  The conclusion questions will summarize PS1.A and the concept of valence electrons.  This knowledge will also prepare students for understanding the Coulombic attraction that occurs between valence electrons and the nucleus.  This concept will be addressed in the synthesis questions on the worksheet.  These questions are not meant to be correct, but just a hypothesis of what will be occurring in subsequent lessons on Coulombic attraction.

Evaluate

10 minutes

Since I don’t collect the cards that students completed, I have them complete the Bohr's model exit slip that has them make a Bohr’s model of one of the first 18 elements.  This is a quick and easy way to assess student understanding (exit slip key) of the content and it allows the students to turn keep their cards for future reference.  The conclusion and synthesis questions will be collected the following day or can be turned in at the end of the period.