One of the most widely recognized scientific symbols is the atom, which shows negatively charged electrons orbiting a dense nucleus made up of positive protons and neutral neutrons. Although physicists still generally agree on this basic atomic model, they also acknowledge that it is incomplete.
At one time, physicists thought that protons, neutrons, and electrons were fundamental particles, meaning that they are not made of anything smaller. However, since the 1950s, more and more evidence has been discovered for the existence of even smaller particles. For example, physicists now know that protons and neutrons, the particles that make up an atom's nucleus, are built from six types of particles, collectively called quarks. In addition, they have found evidence for about a dozen other types of elementary particles, some associated with matter and others with the forces that act on matter. Many physicists surmise that all matter and forces are comprised of these 19 different types of particles.
Today, many physicists subscribe to what is known as the standard model of particle physics. This model classifies all matter particles as either quarks or leptons. In addition, it places all particles responsible for forces into another group, the bosons. By placing all 19 matter and force particles into these three categories, the model explains the properties and behaviors of all particles in the simplest of terms, an inherent objective of all scientific theories.
Whether or not quarks, leptons, and bosons are in fact fundamental is yet to be determined. String theorists think not. According to their calculations, all particles are made up of even smaller vibrating strands of energy, called strings. These strings, if they exist, would only differ from one another in their levels of energy and patterns of vibration, and would therefore be truly fundamental.