This image is a model of an atom
Windows to the Universe original animation by Randy Russell.
What's a molecule?
A Matter of Scale - interactive showing the sizes of things, from very tiny to huge - from NSF
Atoms are the smallest size pieces that elements come in. Each atom has a small but heavy nucleus at its center. A cloud of tiny, fast-moving electrons orbit around the nucleus. The nucleus contains protons and neutrons. Most atoms have about as many neutrons as protons. A "normal" atom has the same number of electrons and protons.
The animation of an atom on this page is NOT to scale. In a real atom, the protons, neutrons, and electrons are very tiny compared to the distance between the nucleus and the electrons. Most of an atom is empty space. Atoms are incredibly small. They have sizes between 60 and 500 picometers (a picometer is 10-12 meters, or one trillionth of a meter!). The nucleus of an atom is about 100,000 times smaller than the whole atom. Almost all (more than 99%) of the mass of an atom is in the nucleus.
Sometimes two or more atoms "stick together" to form a molecule. Some molecules have atoms of just one type. For example, an oxygen molecule (O2) has two oxygen atoms. Other molecules combine different types of atoms. Methane (CH4) has one carbon atom and four hydrogen atoms.
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