\begin{definition}[Definition:Atom (Physics)/Nucleus]
The '''nucleus''' of an atom is the relatively dense core at its center.
It consists (to a first degree of approximation) of a number of protons and a similar number of neutrons.
The number of protons determines the nature of the chemical element which consists of aggregations of instances of the atom.
The number of neutrons determines the isotope of the element.
\end{definition}