How do atoms become isotopes?

Let's take the simplest example, atomic hydrogen, #H#. In hydrogen nuclei there is ONE fundamental, positively charged nucleon, a NUCLEAR proton (note that in this "nucular" context, protons are different from acidic species, which we often represent as #H^+#. We could represent this species as #""^1H#, and in fact most of the hydrogen in the universe (>99%) is this isotope.

However, a hydrogen nucleus could contain a NEUTRON (a fundamental neutrally charged nucleon). This is still a hydrogen nucleus because the elemental definition goes by #Z#, the number of positive nucleons, and #Z# is still #1# for this example. We would represent this as #""^2H#, or as a deuterium nucleus. A smaller quantity of hydrogen atoms in the universe could have 2 neutrons (in addition to the 1 proton present by default!), and we would represent this as the isotope tritium, #""^3H#.

These isotopes are reasonably chemically similar (there are differences but I don't want to make this treatment unnecessarily long and complicated (YEAH, RIGHT!), but they all illustrate the phenomenon of isotopes. Elements (which have the same atomic number by definition) COULD have different masses because their nuclei contain a varying number of NEUTRONS.

Most elements have a few isotopes that are naturally occurring. Chemists can exploit isotopic distribution by the use of isotopic labelling: i.e. doing reactions with different isotopes and seeing where the label (the particular isotope) ends up. Isotopic labels can be interrogated by different means: by mass spectroscopy, where a deuterium label would have the formula mass + 1; or by NMR spectroscopy, #""^2H# #NMR# spectroscopy is very common and is an easy experiment to do.

I used the term NUCLEON in this question to describe a nuclear particle (it is an advanced term but is useful in the context). A nucleon is a PROTON or a NEUTRON that comprise atomic nuclei.