molecule. 1  Chemistry, Physics. the smallest physical unit of an element or compound, consisting of one or more like atoms in an element and two or more different atoms in a compound. 2  Chemistry. a quantity of a substance, the weight of which, measured in any chosen unit, is numerically equal to the molecular weight; gram molecule.
When I do crunches my neck starts to tighten up and it makes me have to rest before doing another set of crunches.
Crunching Sound in Neck. What causes cracking neck? A crunching sound in neck is mostly normal, but can be a sign of arthritis, which usually improves with rest, medications and neck exercises. Some people find it weird that whenever they move their heads, they hear a crunching sound in neck.