When a person steps on something sharp (like a nail), the leg that is stepping on the nail reflexively pulls away (the flexor muscles contract and the extensor muscles relax), while the other leg reflexively takes the weight of the whole body (the flexor muscles relax and the extensor muscles contract) in preparation for the jerk.
A molecule is the smallest particle of a substance that exists independently. Molecules of most elements are made up of only one of atom of that element. Oxygen, along with nitrogen, hydrogen, and chlorine are made up of two atoms. Look at the model of oxygen above. The two balls represents the two oxygen molecules.
Protons Electrons Neutrons Q UESTIO N 19 These are the smallest units that retain all of the properties of their type of matter. Atoms Molecules Matter Chemistry Q UESTIO N 20 This subatomic particle maintains a negative charge. Electron Neutron Proton.