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.
A swollen bump on back of neck can be caused by physical injury of the neck muscle or tendon. If the muscle of the neck are damaged or injured, the part of the muscle that is injured will stick out forming a bump. Injured neck muscle or tendon cause sharp pain when moving the head.