Atomic nucleus

The dense, positively charged center of an atom, composed of protons and neutrons and containing nearly all its mass.

Proton: A positively charged subatomic particle found in atomic nuclei. Its number in a nucleus determines the element’s atomic number.
Radioactive decay: The spontaneous transformation of an unstable atomic nucleus into a more stable configuration. It changes a nucleus without requiring a collision with another particle.
Deuterium: The hydrogen isotope whose nucleus contains one proton and one neutron. Its two-nucleon nucleus is the simplest bound system of a proton and neutron.
Isotope: One of multiple forms of an element with the same proton number but different neutron numbers. Isotopes share nuclear charge but differ in nuclear mass and often stability.
Neutron: An electrically neutral subatomic particle found in atomic nuclei. Neutron number distinguishes isotopes of the same element.
Alpha decay: Radioactive decay in which a nucleus emits a helium-4 nucleus. The parent nucleus loses two protons and two neutrons.
Helium-4: A helium isotope with two protons and two neutrons in its nucleus. Its tightly bound nucleus is a prominent example of nuclear stability.
Nuclear shell model: A model that describes nucleons as occupying quantized energy levels within a nucleus. It accounts for especially stable nuclei with filled proton or neutron shells.
Nucleon: A collective name for either a proton or a neutron. Nuclei are assemblies of these two kinds of particles.
Beta decay: Radioactive decay in which a neutron or proton changes type through the weak interaction. It changes the proton-to-neutron balance while often preserving nucleon number.

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