Atomic Mass Unit; Isotopes, Isobars and Isotones

Because atoms and nuclei are so light, their masses are almost never expressed in kilograms directly; instead, physics and chemistry use a convenient unit of their own, the ATOMIC MASS UNIT (symbol u), defined as exactly one-twelfth of the mass of one atom of the carbon-12 isotope, $^{12}_{6}\text{C}$:

Because mass and energy are equivalent (Section 8.6), it is often more convenient to quote a mass directly in its energy equivalent, and one atomic mass unit works out to be almost exactly $931.5\ \text{MeV}$ (Example 3) -- a conversion used constantly throughout nuclear physics, since it lets a mass difference of even a few thousandths of a u correspond to an energy of several MeV, far more than any chemical reaction can release.

Not every nucleus of a given element has exactly the same number of neutrons, and nuclides that are related to one another in particular ways are given their own names:

• ISOTOPES are nuclides having the SAME atomic number $Z$ (hence the same element, and the same number of protons) but DIFFERENT mass numbers $A$ (hence different numbers of neutrons). Since isotopes have identical proton number, they occupy the same place in the periodic table and share essentially the same chemical behaviour, differing only in mass and in nuclear stability. Ordinary hydrogen, $^{1}_{1}\text{H}$ (protium, no neutron), $^{2}_{1}\text{H}$ (deuterium, one neutron) and $^{3}_{1}\text{H}$ (tritium, two neutrons) are the three natural isotopes of hydrogen.

• ISOBARS are nuclides having the SAME mass number $A$ but DIFFERENT atomic numbers $Z$ -- that is, a different total number of nucleons is arranged differently between protons and neutrons, so isobars belong to DIFFERENT elements entirely, even though their total nucleon count matches. $^{40}_{18}\text{Ar}$ and $^{40}_{20}\text{Ca}$ are isobars: both have $A=40$, but argon has $Z=18$ while calcium has $Z=20$. …