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.
Here is how you do a proper crunch: 1  Lie on your back. 2  Bend your knees to a comfortable position. 3  Lock your fingers behind your head or cross your arms in front of your chest.  Curl your head, shoulders, and upper back off the 1  floor. Keep your lower back in contact with the floor; you should only rise up a few inches.  Breathe out as you rise.