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.
Sit-ups and crunches are a good exercise to help strengthen your core and to flatten your tummy. There are many variations of exercises that can target the obliques and lower abdomen to trim away belly fat and give you a long lean look.