Dear Science,

This question may seem a tad obscure, but it comes up when I wonder if I could have a molecule of water in my body that once passed through, say, Benjamin Franklin, or another famous (or infamous, for that matter) person.

How long do water and other covalently bonded molecules actually remain intact, i.e., with their original hydrogen and oxygen atoms? Are they promiscuous? Do they exchange atoms with other molecules frequently? How could one determine the age or stability of a given molecule?

Musing Over Beer In Ballard

I have sad news, my dear beer drinker in Ballard: There is virtually no chance that a water molecule that passed the lips of Benjamin Franklin (Science's favored founding father, by the way) has passed through your lips. Covalent bonds seem like they should be so solid and enduring, yet they are fleeting—especially in the case of water.

Atoms, in contrast, are generally enduring. Aside from the more vigorously radioactive of elements, atoms retain their identity for eons. That's not to say atoms are universally happy with existence.

Let's think back to high-school chemistry. Atoms are made up of nuclei (composed of protons and neutrons) surrounded by shells of electrons. All atoms wish to have their outermost shell of electrons filled. For helium and hydrogen, the outer shell can hold two electrons. Hydrogen, with a single electron, is only halfway there. All other atoms—including oxygen—need eight electrons to have a full outer shell. (This is why the periodic table has eight columns.) Oxygen has only six electrons in the outer shell, needing two more to fill it out. The covalent bond in water is the grand deal between oxygen and hydrogen atoms: By sharing their electrons, two hydrogen atoms and one oxygen atom can convince themselves they are complete—like their smug cousins helium and neon.

With surprising speed, this arrangement breaks down and re-creates itself. The oxygen atom in a water molecule is chock full of electrons (three pairs, oh my!) begging for a hydrogen ion (a lonely bare proton) to jump from another water molecule and nestle into a sweet pair just waiting. The result of this shifting is a hydronium molecule (three hydrogens and an oxygen) and a hydroxyl molecule (an oxygen and a single hydrogen). This shuffling about occurs in all water, more so when things warm up a bit. With all of this, a given water molecule lives for only about 10 hours.

So it ends up being highly unlikely that a single water molecule—a pair of hydrogens and an oxygen—would remain attached across the centuries. Blame equilibrium, my friend, and the deliciously low disassociation constant of water.

Associatingly Yours,

Science

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