Table 1 A brief review of molecular model teaching tools and our observations on some physical molecular models

Physical molecular models

o Ping-pong balls [3], rubber balloons [4], bottle caps [5, 6], whiteboard markers [7] or other materials [8,9,10,11,12,13,14] (Repurposing available materials)

o Plasticine [15], clay or dough [16], (Hand-building from malleable materials)

o Laser/wire cut [17], 3D printing [18,19,20,21], 3D Laser engraving [22], magnet-embedded silicone [23] (Custom-made models by advanced techniques)

Computer software or web apps

o Specialized online services such as molecular clickers based on 3Dmol.js [24], Android application for chemical OCR and 3D visualization [25], unit cells modelling [26] and molecular shape modelling [27]

o Platonic solids in POV-Ray language [28] or orbital displays in Mathematica [29]

o Molecular graphics software

 o Proprietary commercial software e.g. GaussView [30], Chem3D [31]

 o Open-source software e.g. Avogadro [32, 33], Chimera [34], Gabedit [35], IQmol [36], PyMOL [37] and QuteMol [33]

 o Online service for molecular graphics such as WebMO [38] and CheMagic [39]

3D/augmented reality (Advanced visualization techniques usually require both specific hardware and software for operation.)

o 3D video glasses [40, 41]

o App on mobile devices [42,43,44]

o Holographic projection [45]

● Commercial molecular model sets can be a costly one-time investment. As many pieces are included in a set to represent various atom/bond types, molecular model users usually spend a considerable amount of time for inventory inspection. The time may be reduced if an analytical balance is used for this counting process. In many cases, students also spend a significant amount of time or find it difficult to disassemble models back to the original pieces. This disassembling is usually needed before building the next molecules as a standard molecular model set of 165 pieces contains only 14 carbon atoms.

● Other physical models (such as table tennis balls and plasticine) may have pros and cons. For example, plasticine is malleable and can be used to form ball-and-stick as well as space-filling models. We asked students to weigh plasticine so that they hand-build models that are proportional to actual molecules. Students clearly observed the relative size of hydrogen compared to halogen in hydrogen halides and correctly predicted the acidity trend. However, the use of plasticine can be cumbersome, dirty and distracting for some students.