While matter is commonly understood to be built from atoms, particle physics reveals that atoms are not the smallest fundamental units. The building blocks of the atomic nucleus are composed of even tinier constituents called quarks. Quarks are among the most fundamental particles known to science, forming the structure of all observable matter in the universe.
Defining the Up and Down Quarks
Quarks are elementary particles, meaning they are not made up of any smaller components. They are classified as fermions, a category of particles associated with matter, which also includes electrons. There are six distinct types, or “flavors,” of quarks, but only the up quark and the down quark construct the ordinary matter that makes up our world.
The up and down quarks are the lightest of all quark flavors and constitute the first generation of matter particles in the Standard Model. They are stable compared to their heavier counterparts, which quickly decay into these lighter first-generation particles. Their stability and prevalence make them the primary components for the structure of everyday objects.
How Quarks Build Protons and Neutrons
The up and down quarks combine in specific arrangements to form composite particles called hadrons. Hadrons made up of three quarks are known as baryons, and the most common examples are the particles that reside in the center of an atom. The proton, which carries a positive charge, is assembled from two up quarks and one down quark (UUD).
The neutron, which carries no net electric charge, has a composition of one up quark and two down quarks (UDD). This structural difference—a simple swap of one quark flavor—accounts for the distinction between the two primary components of the atomic nucleus. The unique quark combinations give the proton and neutron their distinct identities and properties within the atomic structure.
Key Characteristics and Interactions
A distinguishing feature of quarks is their fractional electric charge, a property not seen in larger particles like the electron or proton, which have whole-number charges. The up quark carries an electric charge of positive two-thirds (+2/3) of the elementary charge, while the down quark carries negative one-third (-1/3). When combined, these fractional charges sum to the integer charge of the proton (+1) and the neutral charge of the neutron (0).
Quarks possess another fundamental attribute called “color charge,” which is unrelated to visual color and comes in three types: red, green, and blue. This charge is the source of the strong nuclear force, mediated by exchange particles called gluons. Gluons carry color charge, causing the force between quarks to increase as the distance between them grows. This phenomenon, known as confinement, means that quarks are never observed in isolation but are always bound together within color-neutral hadrons.