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.
Of course, crunching or grinding noises during neck motion that is associated with other symptoms could be important. For example, such noises accompanied by neck pain, arm weakness or numbness could be an indication of arthritis, disc disease or a combination of these.