What Is an Atom's Nucleus Made Of? A Look Inside the Core
An atom’s nucleus is made of protons and neutrons, collectively called nucleons. Protons have a positive electric charge, while neutrons have no electric charge. Both are composite particles: each has three valence quarks, bound by the strong interaction. The proton’s valence quarks are two up quarks and one down quark; the neutron’s are one up quark and two down quarks U.S. Department of Energy.
The number of protons determines the element. The number of neutrons determines which isotope of that element the nucleus is. Protons and neutrons stay bound in the nucleus because of the strong nuclear force, which acts between nucleons at very short distances International Atomic Energy Agency.
Protons, neutrons, and quarks
A proton and a neutron have similar sizes, but their charges and valence quark arrangements differ. An up quark has an electric charge of +⅔, and a down quark has a charge of −⅓. In a proton, the charges of two up quarks and one down quark add to +1. In a neutron, one up quark and two down quarks add to zero.
The three-quark description gives the main composition of each nucleon, but it is not a complete picture of everything inside. Protons and neutrons also contain gluons, which carry the strong interaction, along with short-lived quark and antiquark pairs. These particles and fields are part of the nucleon’s internal structure. Quarks are not observed in isolation; they remain confined within composite particles.
The Particle Data Group reviews measurements and properties of quarks and other particles in its Review of Particle Physics. Its reference material describes the particles within the Standard Model, the framework physicists use to describe fundamental particles and their interactions.
Where nucleon mass comes from
The mass of a proton or neutron is not simply the sum of the masses of its three valence quarks. Much of a nucleon’s mass comes from the energy of the strong interaction and the motion of its internal constituents. Because mass and energy are related, this energy contributes to the measured mass.
This means that describing a nucleus as a collection of protons and neutrons is useful, but each nucleon has its own internal structure. At the scale of an atom, the nucleus is a compact assembly of these composite particles.
What holds the nucleus together
Protons repel one another because they have the same positive electric charge. Yet they remain together in many nuclei. At the short distances between nucleons, the residual strong nuclear force attracts protons and neutrons and can overcome the electrical repulsion between protons.
Neutrons contribute to nuclear binding without adding electric repulsion. Their presence can help a nucleus remain bound, although the effect depends on the particular numbers of protons and neutrons. There is no single neutron-to-proton ratio that guarantees stability for every nucleus.
As the number of protons increases, electrical repulsion within the nucleus also increases. A suitable number of neutrons can help bind a nucleus, but some combinations of protons and neutrons are unstable and decay. Nuclear stability depends on how the particles are arranged and on the balance of forces within the nucleus.
One example: carbon-12
Carbon-12 shows how proton and neutron counts describe a nucleus. Its atomic number is 6, so its nucleus contains six protons. The number 12 is its mass number, the total number of protons and neutrons. Subtracting the six protons leaves six neutrons.
The notation can be summarized as:
- Protons: 6
- Neutrons: 6
- Mass number: 12
The proton count gives the element’s identity. A nucleus with six protons is carbon, regardless of how many neutrons it contains. The neutron count distinguishes one carbon isotope from another. For a neutral carbon atom, six electrons balance the positive charge of the six protons. Those electrons are outside the nucleus, so they are not part of its composition.
The mass number is often written as A = Z + N, where Z is the number of protons and N is the number of neutrons. If the mass number and proton count are known, the neutron count is N = A − Z.
Key Takeaways
- An atomic nucleus is made of protons and neutrons, together called nucleons.
- Protons and neutrons are composite particles with three valence quarks each. Gluons and other short-lived constituents are also part of their internal structure.
- The strong nuclear force binds nucleons at short distances. It can overcome the electrical repulsion between protons.
- The number of protons determines the element. The number of neutrons distinguishes its isotopes.
- Carbon-12 has six protons and six neutrons, for a mass number of 12.
