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.
F- has the smallest radius. It has ten electrons, so only the first two 'shells' are filled. The others have electrons in the third 'shell', which is at a greater distance from the nucleus. Sice flourine is in the lowest energy level so it has the lowest radius. fluorine. it comes first in the periodic table out of all of them. so its nucleus has the least amount of protons and neutrons - which have much more mass than any electrons.