= Atomic nucleus
{title2=$A=Z+N$}
{wiki}
= Nucleus
{synonym}
The central system of <protons> and <neutrons> in an atom. Its mass number $A$ is its <baryon number> in ordinary nuclear states; $Z$ is the proton number and $N$ the neutron number.
The <nucleus> contains almost all of an atom's <mass> and carries positive <electric charge> $Ze$, where $e$ is the <elementary charge>. The <atomic number> $Z$ identifies the <chemical element>; changing $N$ at fixed $Z$ gives another <isotope>. The <mass number> $A=Z+N$ 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.
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