Matter is all the stuff that we see, feel, and smell around us. By definition, matter has mass and takes up space. Matter includes substances such as water, wood, rock, metal, plastic, and air, as well as countless other materials. All types of matter are made up of tiny particles called atoms.
Atoms and the sub-atomic particles they're made of are far too small to be seen, even with the most powerful microscope. Despite this, physicists have developed an understanding of the structure of these particles through experimentation and indirect observation.
Models of the structure of atoms have changed a great deal since the first one -- a simple, undifferentiated sphere -- was proposed in the nineteenth century. In today's atomic model, a "cloud" of negatively charged particles called electrons orbits around a small, dense nucleus of positively charged protons and neutral neutrons.
For many decades, physicists thought that protons, neutrons, and electrons were fundamental -- 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, contain even smaller particles called quarks. Scientists have identified six known quarks, which join together in groups of three to create either positively charged protons or neutral neutrons, depending on the specific combination.
One of the most important tools for developing an understanding of atomic structure is the particle accelerator. This device accelerates particles until they move at almost the speed of light and contain a great deal of energy. Their paths are focused so that once their speed and energy are high enough they collide with other such particles. Devices called particle detectors, which surround the acceleration chamber, record the results of the particle collisions. The particle detectors can count the particles, categorize their tracks, measure their energy, record the length of their flight, and differentiate one particle from another. By analyzing all this data, physicists can determine the structure of the original particles.
