Making the Invisible Visible: Modelling Solid Bonding & Brittleness

Teaching Chemistry is about making the invisible visible! One of the connections we try to make in high chemistry is how a compound's atomic structure is related to its physical properties. We introduce physical properties (e.g. brittle, hardness, melting point) and make comparison between different types of solids (e.g. ionic, metallic and covalent solids). This set of models is based upon Flinn Scientific's Models to Illustrate Ionic and Metallic Solids

Supplies

PLA
Round Magnets (Diameter: 18 mm, Thickness: 5 mm)
Optional: Hot glue gun

Step 1: What Is Solid Bonding?

Solid Bonding refers to giant crystal structures made up of repeating parts/subunits. Crystalline solids can be broken down into four main categories:

Ionic solids (e.g. table salt NaCl; marble, CaCO₃),
Covalent/Molecular solids (e.g. ice, rubber),
Covalent network solids (e.g. diamonds, graphite, silicon), and
Metallic solids (e.g. copper, bronze).

These categories are based on the relative strength holding the atoms, molecules or ions together, and correlate with the physical properties of the solids. For more info, click here.

Ionic Solids typically consist of positively-charged and negatively-charged particles. Table salt (sodium chloride, NaCl) might be a familiar example. The metallic sodium ion (Na¹⁺) having a +1 charge and the non-metallic chloride ion (Cl¹⁻) having a -1 charge. Since opposite charges attract, the positive Na¹⁺ and negative Cl¹⁻ ions form sodium chloride (NaCl).

We can use magnets to represent the charged particles. We know that the opposite poles of the magnet (+ and -) will attract and the same poles will repel (+ and + OR - and -). Most importantly, we can FEEL this attractive/repulsive force!!

Now ionic compounds don't just consist of one sodium (Na¹⁺) and one chloride (Cl¹⁻) ion. They actually consists of many repeating copies that arrange themselves in a 3D structure called a crystal lattice structure. How these layers interact can explain their physical properties.

We will focus on brittleness in this model. Brittleness describes materials that are easily broken, damaged, disrupted, cracked, and/or snapped. Glass is brittle, clay is not.

Students often have trouble distinguishing brittleness from:

Hardness: the ability to resist being deformed,
Malleability: the ability to be pressed, hammered or rolled into a sheet, and
Ductility: the ability to be drawn out into a wire.

In general, brittleness is the opposite of ductility, and some of these physical properties are related (e.g. ductility & malleability), BUT compounds can be:

hard and brittle (e.g. ceramics, glass, the snap of properly-tempered chocolate, the steel hull of the Titanic),
hard but not brittle (e.g. most metals, improperly-tempered chocolate),
malleable but not ductile (e.g. aluminium, lead),
etc.

One of the main comparisons we want to students to understand is how brittleness differs in ionic and metallic solids. In general:

Ionic compounds are brittle
Metallic compounds are not

For more info, check out the following videos:

Brittleness of an Ionic Crystal (Excellent video showing the layers shearing off)
What are Metallic Bonds?
Material Properties 101 (Vocabulary: Stiff, Tough, Strong, Ductile, Brittle, Hard)

Step 2: 3D Printing & Embedding the Magnets

I modelled the layout in the Flinn Scientific instructions using Tinkercad: two Ionic solids with different packing patterns and one Metallic solid. The STL and Tinkercad designs are for 18mm disc magnets with a thickness of 5 mm.

Here are the Tinkercad links:

Ionic Model 1 (Rectangular Pattern)
Ionic Model 2 (Triangular Pattern)
Metallic Model

If you want to design your own patterns, I used the Engineering ToolBox: Circles within a Rectangle to determine the circle coordinates for the Triangular pattern.

The models were printed in PLA with 0.2mm Layers & 30% Infill. I used the "Pause at Height" command and inserted the magnets at the appropriate layer (e.g. after 7 mm if your magnets are 5mm thick). If you have trouble with the magnets remaining in the correct configuration, a small amount of hot glue will hold them in place. Double check the polarity of your magnets before restarting the print.

Check out this video on embedding magnets into your 3D prints.

Step 3: Modelling Brittleness in the Classroom

Instructions:

Stack the two parts of Ionic Model 1 together and push the top layer. Show how the layers do not slide smoothly but "jump".
Repeat for Ionic Model 2. Show how the layers still "jump" even though the packing pattern is different.
Repeat for Metallic Model. Show how the layers move slightly but do not "jump".

Discussion: Ionic crystals shatter because the striking/sliding force pushes like charges together. The repulsive force is enough to push the layers apart and the ionic crystal shatters. In the metallic model, striking/sliding results in another stable configuration with little repulsion. The layers are still held together, and the metallic solid does not shatter.