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.
In the neck, this can happen when your joints glide as you turn your head. This is normal. However, if crunching sound in neck is accompanied by pain, consult your doctor to find out if there is any underlying abnormality in the joint. There could be an injury in your ligaments or some loose cartilage.