Atoms make up the universe and also all life. Too small to be seen with the eye or a regular microscope, atoms can stick together to form molecules. A single molecule can be small, built of just two or three atoms, or it can consist hundreds of atoms, like in a strand of DNA, or of trillions of atoms, like in a vulcanized tire.
Vulcanized tire at rubber factory in Canada, 1943. Photo by Harry Rowed. National Film Board of Canada. Photothèque. Library and Archives Canada
There are more than 100 types of atoms recorded in the Periodic Table of Elements. Every atom contains subatomic particles, including protons, neutrons, and electrons.
Protons and neutrons. An atom’s identity (what element it is) relies on the number of protons it contains. Hydrogen always has one proton, while helium always has two, and carbon always has six. Atoms can have roughly the same number of neutrons as protons, but that is hardly the rule because they also often vary according to the specific isotope.
Electrons. The number of electrons can also vary, depending on what the atom is bonded to. Atoms like their electrons to be paired and in specific arrangements, and atoms bond together to stabilize their electron arrangements. Every electron adds a charge of -1, and every proton adds a charge of +1. Therefore, an atom can only have a neutral charge if it has the same number of electron as protons. An atom or molecule with a non-neutral — positive or negative — charge is called an ion.
Under the right circumstances, electrons are the particles that transmit electricity.
Properties of Elements, how stuff acts. Knowing the identity of an atom isn’t enough to tell you how that substance will act. Many factors affect how atoms behave, foremost whether they are bonded to other atoms and how.
A compound is any mix of more than one atom. Like letters of the alphabet that can be combined to form words with very different meanings, the same atoms can be reorganized in a variety of ways to make vastly different compounds with wildly different properties. Even a compound made of a single kind of atom can be varied. For example, carbon atoms can stick together to form diamond, a clear, hard crystalline substance. And carbon atoms can stick together in a completely different way to form graphite, the slippery, silvery stuff of number 2 pencils.