Nuclear Physics

At the centre of every atom sits a nucleus so small that the radius of the whole atom is about $10^{5}$ times the radius of the nucleus. Almost all of the atom's mass, roughly 99.9 percent, is packed into this tiny volume, and it carries the entire positive charge of the atom. The nucleus is built from nucleons, and nucleons come in two kinds: protons, each with a positive charge of $1.6 \times 10^{-19}\text{ C}$ and a mass of $1.673 \times 10^{-27}\text{ kg}$, and neutrons, which carry no charge and have almost the same mass, $1.675 \times 10^{-27}\text{ kg}$.

The tidy language for this is worth fixing early. The number of protons is the atomic number, also called the charge number, written $Z$; the total charge of the nucleus is $Ze$. The mass number $A$ is the total count of nucleons, protons plus neutrons together, not protons alone. The neutron number follows by subtraction: $N = A - Z$. Because a whole atom is electrically neutral, the number of protons in the nucleus equals the number of orbiting electrons. Heavy nuclei need proportionally more neutrons for stability, so in the lighter elements neutrons and protons are roughly equal in number, while in the heavier ones the neutrons outnumber the protons. Uranium-235, written $^{235}_{92}\text{U}$, carries $92$ protons and $235 - 92 = 143$ neutrons.