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.
You should be doing crunches. Sit-ups focus on the muscles of the back, with very little emphasis on the chest and abs. It is true that they do affect those areas, but the change is slight, and therefore sit-ups are ineffective for that purpose.