Question: Why do certain objects shatter? What are their structures like? Why, also, are some things able to bend?

Rob Temperton answered on 18 Jun 2015:

I think I know why some plastics shatter and others don’t. It is because plastics are made of long molecular chains. These can sometimes by stretchy. If for example such an object were to fall and it the floor, some of the energy of the impact goes into stretching the chains – so sort of absorbing the energy away from the point of impact. How shatterable a material is depends on how stretchy the molecular chains are. This links into the second factor – how easily the chains can slide past each other (what good is stretching if there is no where to stretch to?!). Materials that allow the chains to slip and slide a little bit are less prone to shattering.

Materials like glass are more complex – glasses are weird but awesome. They sort of lye half way between a solid and a liquid. When warm, the molecules flow past each other like a liquid. When cool, they get locked in place but do not form nice crystal structures. This can leave weak points in the material. As an object receives an impact, the stress propagates through the material and fails at a structurally weak point.

If you go to the extreme and take a perfect crystal of a material like silicon, you will find it has a tendency to crack in certain directions. These directions are determined by the structure of the crystal (the pattern made by the atoms) where it takes less energy to break bonds in some directions rather than others.

Bending also depends on the type of material. Some plastics like rubber are stretchy (as discussed before) so will bend back it I shape when you remove the force you apply.

Metals generally bend more easily than non metals, again because of bonding. Metals that bond via metallic bonding (a sea of delocalised electrons acts as a sort of clue between nuclei) are more flexible than purely covalent (e.g silicon) or ionic (e.g salt) structures. However, they do not bend back on their own – there is not that elastic restoring force that exists with rubbers.

So I guess, the most general answer is that it comes down to bond strength. Whether that be intermolecular bonds between polymer chains being weak and allowing them to slide past eachother, or atomic bonds in a crystal, it is the bonding that ultimately determines how a material shatters or bends.

Does that make sense?

PS – sorry for the Long answer… I may have got a bit carried away!