Matter is all the "stuff" that we see, feel, and smell around us. By definition, matter has mass and takes up space. It includes substances like water, wood, rock, metal, plastic, air, and countless other materials. All types of matter are built from tiny particles called atoms.

Atoms and the subatomic particles they're made of are far too small to be seen, even with the most powerful microscopes. Despite this, physicists have developed an understanding of their structure using experimentation and indirect observation.

Models of the structure of atoms have changed a great deal since the first -- a simple, undifferentiated sphere -- was proposed in the nineteenth century. In today's atomic model, a "cloud" of orbiting, negatively charged particles called electrons surrounds a small, dense nucleus of positively charged protons and neutral neutrons.

For many decades, physicists thought that protons, neutrons, and electrons were fundamental, which means structureless, or not made of anything smaller. While it appears that electrons are, in fact, fundamental, physicists now know that protons and neutrons, the particles that make up an atom's nucleus, are built from even smaller particles called quarks. Scientists have identified six known quarks, which combine in groups of three to create either positively charged protons or neutral neutrons, depending on their combination.

Most elements have both a stable form and one or more unstable forms. An element is stable when its subatomic particles are in a particular numerical balance. For example, a carbon nucleus with six neutrons and six protons is stable, meaning that it is unlikely to give off a particle to improve its stability. An atom with too many or too few neutrons in relation to its number of protons will be radioactive, meaning that it is likely to undergo radioactive decay, a process in which the nucleus gives off high-energy particles. Sometimes a radioactive element gives off a proton, changing its elemental identity in the process.