Protons and neutrons form the core, or nucleus, of every atom, except for the most common form of hydrogen. These particles, collectively known as nucleons, account for nearly all of an atom’s mass. Protons carry a single positive electrical charge, while neutrons are electrically neutral; the number of protons determines an element’s identity. Although once considered fundamental and indivisible, advances in particle physics have revealed that protons and neutrons are composite particles built from even smaller constituents.
The Building Blocks of Matter: Introducing Quarks
The elementary particles that make up protons and neutrons are called quarks, which are fundamental constituents of matter in the Standard Model of particle physics. Quarks are unique because they possess a fractional electrical charge, unlike the integer charges of electrons and protons. Scientists have identified six distinct types, or “flavors,” of quarks:
- Up
- Down
- Charm
- Strange
- Top
- Bottom
Only the two lightest flavors, up and down quarks, are stable enough to form ordinary matter.
The up quark carries a fractional positive charge (+⅔), and the down quark carries a fractional negative charge (–⅓). An isolated quark has never been observed in nature because quarks are perpetually confined within the composite particles they form, such as protons and neutrons.
Specific Quark Configurations
Protons and neutrons belong to a class of composite particles called hadrons, and more specifically, they are baryons because they are each composed of three quarks. The precise combination of up and down quarks determines the identity and charge of the resulting nucleon.
The proton’s composition is two up quarks and one down quark (uud). The fractional charges sum to produce the proton’s observed charge: (+⅔) + (+⅔) + (–⅓) = +1. The neutron is built from one up quark and two down quarks (udd). Summing the neutron’s charges results in a net charge of zero: (+⅔) + (–⅓) + (–⅓) = 0. This combination explains why the neutron is electrically neutral. These specific three-quark combinations are stable, providing the basis for the nuclei of all elements heavier than hydrogen.
The Force That Binds the Nucleus
The strong nuclear force holds these quark combinations together. This force is mediated by exchange particles known as gluons, which act as the “glue” that binds the quarks within the nucleon. Gluons are constantly exchanged between the quarks, and this intense interaction is described by the theory of quantum chromodynamics (QCD).
Quarks possess a property called “color charge,” which is analogous to electrical charge but comes in three types, arbitrarily named red, green, and blue. The strong force requires that all observable composite particles, like the proton and neutron, must be “color neutral,” often referred to as being “white.” This color neutrality is achieved when the three quarks inside a baryon each carry a different color charge.
The force holding the quarks together exhibits color confinement. Unlike the electromagnetic force, which weakens with distance, the strong force between quarks increases as they are pulled apart. If one attempts to separate a quark from its partners, the energy required becomes so immense that it spontaneously creates a new quark-antiquark pair from the vacuum energy. This newly formed pair immediately bonds with the original quark, preventing any single, isolated quark from ever being observed. The strong force ensures protons and neutrons maintain their stable three-quark composition. Residual effects of this force also help bind protons and neutrons together to form the atomic nucleus, overcoming the electrical repulsion between the positively charged protons.