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.
As you have concluded, popping your neck and back will have no effect on your general health. When your muscles are tight and your spine is stiff, popping your back may provide relief by temporarily relaxing muscles and improving range of motion.