Making the Invisible Visible: Modelling Solid Bonding & Brittleness




Introduction: Making the Invisible Visible: Modelling Solid Bonding & Brittleness

About: Crafychemist and mathemagician extraordinaire. Igniting that spark in STEAM education one project at a time.

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


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:

  1. Ionic solids (e.g. table salt NaCl; marble, CaCO₃),
  2. Covalent/Molecular solids (e.g. ice, rubber),
  3. Covalent network solids (e.g. diamonds, graphite, silicon), and
  4. 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:

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:

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


  1. Stack the two parts of Ionic Model 1 together and push the top layer. Show how the layers do not slide smoothly but "jump".
  2. Repeat for Ionic Model 2. Show how the layers still "jump" even though the packing pattern is different.
  3. 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.

Step 4: Next Steps

Can we model Molecular & Covalent Network Solids as well?

Some ideas:

  • Other magnetic molecular solids:
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    10 months ago

    First: Awesome teaching tool! For visually impaired usable as well! Thanks for this! :)

    May i give an idea on making them even cooler looking?
    - Swap the filament for the positive and negative signs and the circles. Something light like gray or white. Would make it stand out MUCH more...
    - Maybe even go the whole 9 yards and print the base in black and the signs in red and blue? Awesomness overload!
    - If you dont want to switch filaments: after printing, place them face down on wet paint (white) to just color the top edges of the signs and font.

    A small bugfix thou: The video doesnt play:
    "Video unavailable Playback on other websites has been disabled by the video owner."


    Reply 10 months ago

    Very cool! Thanks for the suggestions! I haven't tried the filament swap yet, but it's on my "to-do" list.
    Thanks for the heads-up about the video - I enabled the video embedding so it should work now (fingers-crossed).


    Reply 10 months ago

    Video works. :)