Composition and Size of the Nucleus

The nucleus of an atom is made up of two kinds of particles, collectively called NUCLEONS: PROTONS, which carry a positive charge of exactly $+e$ each, and NEUTRONS, which carry no electric charge at all. A given nuclide -- a nucleus of a specific composition -- is written in the standard notation $^{A}_{Z}X$, where $X$ is the chemical symbol of the element, $Z$ is the ATOMIC NUMBER (the number of protons, which also fixes the element and, in a neutral atom, equals the number of orbiting electrons), and $A$ is the MASS NUMBER (the total number of nucleons, protons plus neutrons). The number of neutrons, sometimes given its own symbol $N$, then follows simply as

Because almost the entire mass of an atom resides in its nucleons (an electron's mass is only about $1/1837$ of a proton's mass), the mass number $A$ also gives, to a very good approximation, the mass of the atom measured in atomic mass units (Section 8.3).

Experiments that fire fast alpha particles or high-energy electrons at nuclei -- essentially repeating Rutherford's original scattering idea with far greater precision -- show that a nucleus behaves, to a good approximation, like a small sphere of nearly uniform density, whose radius $R$ grows with the number of nucleons $A$ it contains according to the simple empirical law

$R = R_0\,A^{1/3}, \qquad R_0 \approx 1.2\ \text{fm} \; (1\ \text{fm} = 10^{-15}\ \text{m})$ …