12.1 Structure and Properties of the Nucleus

12.1 Structure and Properties of the Nucleus

The atomic nucleus comprises two types of particles: protons and neutrons. A proton is the nucleus of the simplest atom, hydrogen, called a protium. The proton has a positive charge of the same magnitude as that of the electron ($1.6 \times 10^{-19}\,\mathrm{C}$) and its mass is $1.67 \times 10^{-27}\,\mathrm{kg}$. The neutron, whose existence was pointed out in 1932 by James Chadwick, is electrically neutral as its name suggested and its mass is nearly the same as that of proton. Now we can say that the nucleus has two types of particles (neutrons and protons) called nucleons.

Besides hydrogen, the nuclei of all other elements consist of both neutrons and protons. The different nuclei are called nuclides. The number of protons in a nucleus is called the atomic number represented by the symbol $Z$. The total number of nucleons, the sum of neutrons and protons, is represented by the symbol $A$ and is called the atomic mass number, or simply mass number. It is written as:

where $N$ represents the neutron number.

In order to specify the given nuclei, the symbol $X$ is commonly used as ${}^{A}_{Z}X$, where $X$ is the chemical symbol for the element. To indicate the mass of atomic particles, instead of kilogram, unified mass scale $(\mathrm{u})$ is generally used. By definition $1\,\mathrm{u}$ is exactly one twelfth the mass of carbon-12 atom ($1\,\mathrm{u} = 1.6608 \times 10^{-27}\,\mathrm{kg} = 931\,\mathrm{MeV}$). Using this value of $\mathrm{u}$, the mass of a proton is $1.007276\,\mathrm{u}$ and that of a neutron is $1.008665\,\mathrm{u}$ while that of an electron is $0.00055\,\mathrm{u}$.

For a particular atom (e.g., carbon), nuclei are found to contain different numbers of neutrons, although they all have the same number of protons. For example, carbon nuclei always have $6$ protons, but they may have different number of neutrons. Nuclei that contain the same number of protons but different numbers of neutrons are called isotopes.