2.1.1
The "79" in \(^{79}\text{Br}\) represents the mass number of the isotope, which is the sum of the number of protons and neutrons in the nucleus of the bromine atom. In this case, \(79\) indicates that the bromine atom has a total of \(79\) nucleons (protons + neutrons).
2.2
An isotope is defined as a variant of a chemical element that has the same number of protons (and thus the same atomic number) but a different number of neutrons, resulting in a different mass number. Isotopes of an element share the same chemical properties but may have different physical properties, such as stability and mass.