Isotopic Varieties of H2O: How Many Kinds of Water are In Your Glass?

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Glasses filled with cold water

What kind of water is in your glass? Image by rollingroscoe.

The woman of the house comes out of her garden. Having worked for hours, she has a “powerful” thirst that only a glass of water can quench. She opens the freezer door, clinks a few cubes of ice into her glass, and then turns to the sink and fills the voids between the cubes with pure water from the tap.

Water consists of two hydrogen atoms and one oxygen atom – H₂O. At first thought, it would seem there should only be one kind of water. This is not accurate, however. That is so because there are isotopes of both hydrogen and oxygen.

To understand what an isotope is, it is important to realize that the number of protons in the center or nucleus of an atom defines an element such as hydrogen or oxygen. All hydrogen has one proton with its plus charge. Outside the nucleus, to balance that electrical charge, an atom of hydrogen has one electron with its negative charge.

However, the nucleus can also contain a specific number of electrically uncharged neutrons. The neutron has a mass not much different from that of a proton. So the addition of one or more neutrons increases the mass of the atom, without changing the hydrogen atom into another element. It continues to be hydrogen, only it is heavier.

Thus, there are actually 18 stable different kinds of water. The explanation is simple.

Hydrogen Isotopes

Most hydrogen atoms contain no neutrons. A form of hydrogen that has an atomic mass of 1 is called protium (¹H). Hydrogen with one proton and one neutron, having an atomic mass of 2, is called deuterium (²H). Tritium (³H) possesses two neutrons per nucleus. Tritium is radioactive. However, tritium’s half-life is about 12.3 years. So it is considered relatively stable.

What is in a name? An oxygen isotope by any other name would provide water as sweet... Copyright image by Decoded Science, all rights reserved.

What is in a name? An oxygen isotope by any other name would provide water as sweet… Copyright image by Decoded Science, all rights reserved.

Oxygen Isotopes

As is true for hydrogen, oxygen occurs as an assortment of isotopes. However, only three of these isotopes are considered to be stable: ¹⁶O, ¹⁷O, and ¹⁸O. Other radioisotopes of oxygen are quite short-lived.

Isotope Kinds of H₂O

At first it might seem that kinds of water with the chemical structure H₂O based on isotope differences only, are 9 in number:

¹H₂¹⁶O, ²H₂¹⁶O, ³H₂¹⁶O,
¹H₂¹⁷O, ²H₂¹⁷O, ³H₂¹⁷O,
¹H₂¹⁸O, ²H₂¹⁸O, and ³H₂¹⁸O

However, there are actually 18 differing isotope kinds of water. This is because each of the two hydrogen atoms in a particular molecule can be a different isotope, giving these 9 additional varieties of water:

¹H²H¹⁶O, ¹H³H¹⁶O, ²H³H¹⁶O,
¹H²H¹⁷O, ¹H³H¹⁷O, ²H³H¹⁷O,
¹H²H¹⁸O, ¹H³H¹⁸O, and ²H³H¹⁸O

Water Composition Percentages

Of course, an ordinary glass of water from the tap does not contain equal amounts of the 18 stable kinds of water. The percentages of each vary considerably. Protium makes up about 99.9885% of all hydrogen. Deuterium makes up 0.1150%. Tritium is so scarce (which is good, since it is highly radioactive) it can be discounted. If we remove it from consideration altogether, this means there are now only 6 kinds of water to consider.

The most prevalent form of oxygen in nature is ¹⁶O, at 99.7570%. ¹⁷O constitutes a mere 0.0380% of the total. ¹⁸O represents 0.2050%. This leads to a chart of percentages of each form of water found in nature. Tritium is so inconsequential, it is listed at 0.0000 percent.

The work of mathematical conversion was accomplished by mathematician Mike DeHaan, for whose efforts I am grateful. In fact, the image below is derived from his enumeration spreadsheet.

Snapshot of Excel Chart by Mike DeHaan, deriving percentages of water varieties from percentages of isotopes of atoms.

Snapshot of Excel Chart by Mike DeHaan, deriving percentages of water varieties from percentages of isotopes of atoms. Copyright image by Mike DeHaan, all rights reserved

Water in an 8-Ounce Glass

An 8-ounce glass of water taken from the average tap contains the following quantities of 6 kinds of water in order of descending quantity:

7.9787 fl.oz. of ¹H₂¹⁶O
0.0164 fl.oz. of ¹H₂¹⁸O (1 drop)
0.0030 fl.oz. of ¹H₂¹⁷O (1/6th of 1 drop)
0.0008 fl.oz. of ¹H²H¹⁶O (1/25th of 1 drop)
Less than 1/1,000,000 fl.oz. of ¹H²H¹⁸O (negligible)
About 1/30,000,000,000 fl.oz. ¹H²H¹⁷O (negligible)

Water: The Drink That Refreshes

As much as many people may like to think another beverage better accomplishes the purpose, water is the drink that refreshes. Only water, as I’ve shown here, is not quite as simple as most of us suspected. It is actually six drinks in one.

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