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.
What causes neck cracking and popping? The snapping or grinding of your neck may be caused by: Synovial fluid changes. The synovial fluid between your joints lubricates movement, and when the pressure changes in this fluid, it creates gaseous bubbles. According to a 2015 study, the creation of these bubbles makes a cracking or popping sound.