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.
This is not exactly accurate; while atoms are matterâthe smallest pieces of matter you can findâmolecules are the smallest bits of compounds. And particles arenât matter, but the building blocks of it. For example, copper is an element. A copper atom is the smallest piece of copper that exists. Hydrogen is also an element; two hydrogen atoms and one oxygen atom combine to form a molecule of water, which is a compound. While atoms are the tiniest bits of matter, they are made of the sub-atomic building blocks of matterâprotons and electronsârevolving around a nucleus.