What is happening on a molecular level when an elastic stretches? Why are some substances elastic but others are not?
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Depending on the bond and the molecular geometry, molecular bonds can bend a bit (even “rigid” steel has a noticeable sag over a long span, and thus can be used to make springs). Past a certain point, the molecules in something like steel flow past each other and don’t go back to their previous position. Brittle materials like glass have very little ability to flex and shatter instead.
Some molecules (like elastic polymers) are long chains that are folded and held in a specific position by weaker electromagnetic bonds between adjacent parts of the molecule that can be overcome, and this allows the folded molecule to take on a different folded shape without breaking any of the stronger covalent bonds. But when the pressure is released, the polymer chain tends to fold up into the initial position.
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