Matt Strassler [December 14, 2012]
Now here’s a remarkable fact, with enormous implications for biology. Take any isotope of any chemical element with atomic number Z. If you take a collection of atoms that are from that isotope — a bunch of atoms that all have Z electrons, Z protons, and N neutrons — you will discover they are literally identical. [A bit more precisely: they are identical when, after being left alone for a brief moment, each atom settles down into its preferred configuration, called the “ground state.”] You cannot tell two such atoms apart. They all have exactly the same mass, the same chemical properties, the same behavior in the presence of electric and magnetic fields; they emit and absorb exactly the same wavelengths of light waves. This a consequence of the identity of their electrons, of their protons and of their neutrons, which will be discussed later.
That all atoms of the same isotope are identical, and that different isotopes of the same element have nearly identical chemistry, is a profound fact of nature! Among other things, it explains how our bodies can breathe oxygen and drink water and process salt and sugar without having to select which oxygen or water or salt or sugar molecules to consume. Contrast this with what a construction company has to do when building a house out of bricks, or out of concrete blocks. Bricks and concrete blocks vary, and are sometimes defective, and so a builder must exercise quality control, to make sure that cracked or over-sized or misshapen bricks and blocks aren’t used in the walls of the house. No such quality control is generally needed for our bodies when we breathe; any oxygen atom will do as well as any other, because we only need the oxygen to make molecules inside our bodies, and chemically all oxygen atoms are essentially the same. (This is all the more true since, for most elements, one isotope is much more common than the rest; for example, most hydrogen atoms [one electron and one proton] have no neutrons, and most oxygen atoms [eight electrons and eight protons] have eight neutrons.)