How do atoms become isotopes?

Question

Owen Ambrose · Accepted Answer

By having a different quantity of neutrons in their nuclei.

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#. Even though there are some differences between these isotopes—I don't want to take too long or complicate this treatment—they all serve as good examples of the isotope phenomenon. Elements, which by definition have the same atomic number, COULD have different masses because of the different number of neutrons in their nuclei. There are a few naturally occurring isotopes for most elements. The most common way for chemists to take advantage of isotopic distribution is through isotopic labelling, which involves performing reactions with various isotopes and observing where the label (the specific isotope) ends up. There are two ways to examine isotopic labels: mass spectroscopy, where a deuterium label would have the formula mass + 1, or NMR spectroscopy, which is a popular and simple experiment. A nucleon is a proton or neutron that makes up an atomic nuclei. I used the term "nucleon" to refer to a nuclear particle in this question (it's an advanced term, but it makes sense).

Cooper Adams · Answer

undefined When two atoms have the same number of protons but different numbers of neutrons in their nuclei, they are called isotopes; as a result, even though they belong to the same element, isotopes have different atomic masses.