The atomic nucleus is the small, dense region at the center of an atom that contains most of the atom's mass. It is composed of two types of subatomic particles: protons and neutrons. - **Protons** are positively charged particles, and their number determines the atomic number of an element, which defines the element itself (e.g., hydrogen has one proton, while carbon has six). - **Neutrons** are neutral particles, meaning they have no charge.
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The central system of protons and neutrons in an atom. Its mass number is its baryon number in ordinary nuclear states; is the proton number and the neutron number.
The nucleus contains almost all of an atom's mass and carries positive electric charge , where is the elementary charge. The atomic number identifies the chemical element; changing at fixed gives another isotope. The mass number counts nucleons, rather than specifying the exact nuclear mass: nuclear binding energy lowers the mass of a bound system relative to its separated constituents. The simplest nucleus is a single proton; other atomic nuclei can contain many protons and neutrons bound by the nuclear force.
A nuclear quantum state has properties such as spin and a spatial nuclear charge distribution. These are different questions from merely counting nucleons. A small-angle elastic scattering measurement can reveal a nuclear charge radius, while a wider range of momentum transfer tests the distribution's shape. The Hofstadter nuclear electron scattering experiment made this connection between scattering patterns and nuclear structure experimentally concrete.
