Composition of nucleus

The nucleus at the centre of every atom is built from two kinds of particles, collectively called nucleons: protons, which carry a positive charge $q=+e$ (equal in magnitude to the electron's charge), and neutrons, which are electrically neutral ($q=0$). The number of protons in the nucleus is called the atomic number, denoted $Z$; the number of neutrons is called the neutron number, denoted $N$; and the total number of nucleons (protons plus neutrons) is called the mass number, denoted $A$, so that

A proton's mass is $1.6726\times10^{-27}$ kg (about 1836 times the mass of an electron), while a neutron is very slightly heavier, at $1.6749\times10^{-27}$ kg.

The standard notation used to specify any particular nucleus is

where $X$ is the element's chemical symbol, $A$ is the mass number, and $Z$ is the atomic number. For example, the nitrogen nucleus is written ${}^{15}_{7}N$, meaning it has 15 total nucleons, of which 7 are protons ($Z=7$) and the remaining $N=A-Z=8$ are neutrons; since the chemical symbol already fixes $Z$ uniquely, the subscript $Z$ is often dropped and the nucleus is written simply as $^{15}N$, read as "nitrogen fifteen." …