Summary

In this activity, students will create particle models of substitutional and interstitial alloys, then engage in discussion to analyze and evaluate the models created by their peers.

Grade Level

High School

AP Chemistry Curriculum Framework

This activity supports the following units, topics, and learning objectives.

• Unit 2: Compound Structure and Properties
  • Topic 2.4: Structure of Metals and Alloys
    • 2.4.A: Represent a metallic solid and/or alloy using a model to show essential characteristics of the structure and interactions present in the substance
• Science Practice 4: Model Analysis
  • Skill 4.C: Explain the connection between particulate-level and macroscopic properties of a substance using models and representations.

NGSS Alignment

This activity will help prepare your students to meet the performance expectations in the following standards:

• 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.
• Science and Engineering Practices:
  • Obtaining, Evaluating, and Communicating Information
  • Developing and Using Models

Objectives

By the end of this activity, students should be able to:

• Use relative sizes and colors to represent different particle identities, along with their arrangements in both substitutional alloys and interstitial alloys of varying composition.
• Use content-specific vocabulary to interpret and evaluate alloy models created by classmates.

Chemistry Topics

This activity supports students’ understanding of:

• Alloys
• Metallic bonding
• Atomic Radius

Time

Teacher Preparation: 15-30 minutes

Lesson: 45-80 minutes

Materials

(Designed for 8 groups, reusable for multiple class sessions)

• Paper plates or other circles (See Teacher Notes for details)
  • 20 large circles in each of 4 different colors (80 large circles total)
  • 80 medium circles in each of 4 different colors (320 medium circles total)
  • 20 small circles in each of 3 different colors (60 small circles total)
• Assorted markers, crayons, or colored pencils to allow students to sketch models (optional)

Safety

• No specific safety precautions need to be observed for this activity.

Teacher Notes

• Start with an introduction/review using the corresponding PowerPoint presentation (available to download).
• Notes are provided with each slide of the presentation.
  • Nature of metallic bonding
  • Definition of alloy (also describes substitutional and interstitial alloys)
  • Uses and limitations of particle models
• The Alloy Description Cards (available to download) should be cut out (A – H) and a one lettered description distributed to each group. The letters are just assigned for ease of discussion of the models.
• Give groups about 5 minutes to choose circle colors/sizes and make models to represent their assigned alloys (one per group). Note that the student handout has the alloys listed in a different order than the A-H assignments.
• Once the models are completed, distribute the student handout and use it to guide students through a gallery walk.
  • In their groups, students should examine, discuss, and attempt to identify all eight alloy models (including their own). With productive discussion, this can take 15 minutes or more, but students may need to be directed/encouraged to spend time discussing. Students should revisit models as needed to distinguish between them, as some have similar compositions. (See Answer Key for discussion/critique of individual models.)
• Teacher-facilitated summary:
  • Question students about their identifications and justifications for each model, one at a time. Use student responses to facilitate discussion about interpretations and misconceptions that arise. (See Tips and Hints below as well as the Answer Key for examples.)

Tips and Hints:

• Choosing circle sizes: As a general requirement, circles within an interstitial alloy model should be different enough in size so that the smaller circles can fit nearly or completely within the space between the larger circles while most of the larger circles remain touching. Circle sizes within substitutional alloy models should be able to take each other’s place without significant distortion of the overall lattice (consult Answer Key document for visual reference).
• Acquire paper circles in one of the following ways:
  • Option 1: Purchase paper plates or circles. Circles can be reused from class to class at least a few times. Below are example descriptions for the circles shown in the Answer Key, as purchased from Amazon for ~$30 total:
    • Construction Paper Circle, 300 Sheets Assorted Colored Craft Paper 6 Inches
    • 500Pcs Cutouts Paper Circles, 3.9 Inch Assorted Colorful Circle Papers
    • 1000 Pieces Tissue Paper Circles, 1 Inch Round Tissue Paper Multicolor Table Confetti Dots
  • Option 2: Make your own circles: If necessary, the teacher could make a variety of sizes of circles on paper and photocopy them onto colored paper. This would require the teacher or students to cut them out before using them, adding to preparation time.
• Find a space large enough to lay out all circles in organized piles. Leave a few minutes of time after the model discussion so you can direct students to return the paper circles neatly before they leave the class.
• Whether you are reviewing the identifications in groups or as a full class, the summary is an important step! Be sure to discuss any identifications were a challenge and address any attributes of the models that students were misinterpreting (incorrect composition, atoms not touching as a solid, incorrect relative atomic radii, etc.) Focus on the following points:
  • Adherence to appropriate modeling for particle-level characteristics
    • Atoms are touching (most alloys are solids) and have a mostly ordered arrangement.
    • Relative circle sizes are justified based on qualitative patterns of atomic radius that can be inferred by comparing placement of each atom on the periodic table.
    • Relative proportion of atoms is accurate (within reason for limited # of circles)
  • Visual differences between the eight alloy models. Discuss how alloys of similar composition could be distinguished from each other.
    • For example, tantalum-carbon alloy and tin-lead solder both have 60/40 composition ratios but can be differentiated by relative sizes of the atoms.
  • Properties of alloys: discuss some of the alloy properties noted in the descriptions, as compared to the properties of individual elements that comprise them. Alloys are only useful if their properties have some benefit over those of the pure elements.
• For students who are absent, you could post the student handout along with photos of the alloys that were created during your class or photos from the Answer Key. You could also use these photos as part of a formative assessment with the lesson or for later reference in a review or quiz.
• If you will have more than 8 groups, you can either find additional or assign some of the alloys to more than one group. If you duplicate alloy assignments, tell students how many are duplicated, but don’t tell them which ones.

Additional background for teachers:

• Metallic bonding: The valence electrons around the nuclei of metal atoms are loosely attracted to all other nuclei in a sample of that metal. This creates a “sea of electrons” that attract and are attracted by all the metal atoms. Since the electrons act as a large group, rather than interacting with only specific atoms, this delocalized attraction leads to common metallic properties such as malleability and electrical conductivity.
• Any substance classified as a metal or an alloy is described as having metallic bonding, with a sea of electrons attracting in a delocalized manner to all nearby atom cores:
  • pure elemental metal (all atoms are of the same element)
  • alloy containing multiple metals (different types of atoms)
  • alloy composed primarily of metals with a small amount of nonmetal (different types of atoms)
• The proportion of atoms in an alloy is not defined by interactions between individual atoms, so it can vary widely. All atoms retain their own atomic structures and are not modified as in ionic or covalent bonding. Alloys are, thus, classified as mixtures.
• Though it may be possible to create mixtures of any two or more types of metals, not all combinations yield a useful result. “Alloy” is the classification assigned to metal mixtures that have been determined to have properties that are “better” in some way than those of the individual components.
• Due to differing atomic properties, such as electronegativity and atomic radius, the net attraction between the metal core lattice and the sea of electrons will vary with differing ratios of atom types. This results in differing physical properties, like malleability and conductivity, and differing chemical properties, like resistance to corrosion, even though alloys are classified as mixtures.
  • Example: Incorporating carbon into a lattice of iron atoms improves strength because the carbon atoms add additional attractive force to attract the sea of electrons and take up interstitial space between iron atoms, thus hindering the overall malleability of the sample. Both the iron and the carbon retain their atomic forms within the sample, so there is no chemical change, therefore making this a mixture rather than a compound.
• Two useful types of alloys are substitutional alloys and interstitial alloys.
  • Substitutional - atoms of one element replace some atoms of another element of similar radius.
  • Interstitial – atoms of one element fill in the spaces between much larger atoms of another element.
  • There is no defined size difference that leads to one or the other. A good guideline is that it is interstitial if there are small atoms taking up space that would mostly otherwise be unoccupied.