A Staff Report from the Straight Dope Science Advisory Board

Why is oxygen usually in the form O2?

January 25, 2001

Dear Straight Dope:

Why is breathable oxygen O2? I know that O3 is ozone and is toxic. Whatever happened to O1? Why does it have to be in a bond of two oxygen molecules for us to be able to use it?

This question isn't easy to answer, but that never stopped us at the Straight Dope Science Advisory Board. Grab a drink, sit down, and try to follow along.

First of all, we need to understand something about atomic structure. For the purposes of this article, the simple classic model will do, where protons and neutrons occupy the center of the atom (known as the "nucleus") and electrons buzz around them in a cloud (known as "orbitals"). Protons are positively charged particles and neutrons have no charge, giving the nucleus an overall positive charge. Electrons have a negative charge. In an atom with an overall charge of zero, there are as many electrons as protons. Stay awake, the answer's coming up.

The number of protons determines what element an atom is. If we could somehow add a proton to a nucleus, we'd change the atom into a different element. For example, carbon, familiar to us in the form of graphite and diamonds, has twelve protons. If we could add a proton to the nucleus, there'd be thirteen, which would make it nitrogen, a clear gas that comprises about 75% of our atmosphere. (OK, that was simplified, but we won't go into isotopes here.) Needless to say, adding or subtracting protons is rare. On the other hand, electrons come and go like hookers at a Democratic Convention.

The electrons are organized into orbitals, or shells, that can be thought of as surrounding the nucleus like nested Russian dolls. For the most part, it takes eight electrons to fill an orbital. Exception: the first orbital, which takes only two electrons.

Atoms like to have their orbitals filled to capacity, even if by rights they don't have enough electrons to do so. (Atoms don't really "like" and "want" things, but let's not get technical.) For example, hydrogen, with one proton and one electron, wants one more electron, which would fill the first orbital with two electrons. How to come up with another electron? There's a simple, elegant answer: sharing. Two hydrogen atoms each put their electrons into a "common area" so both can claim them. Voila, each atom now has a filled first orbital, even though overall there's only one electron per atom. What's more, the two atoms are now bonded together to maintain this stable arrangement. This is called a "covalent bond" and is the way most atoms stick together. Since each hydrogen atom had to "borrow" one electron to fill its orbital, this is known as a single covalent bond. The resultant molecule is called H2.

Oxygen has six electrons in its outermost orbital, two shy of eight. It wants two more. To get them, the oxygen atom will "share" two of its six electrons with another oxygen atom. In other words, each oxygen atom keeps four electrons but puts two into the "common area." Since each oxygen atom antes up two electrons, there's a total of four in the "common area," which each oxygen atom gets to claim, filling its outer orbital. Since each atom had to borrow two electrons, this is called a double covalent bond. A double bond is stronger than a single bond and it's why most free oxygen is found in the form O2.

Another example is nitrogen, which has five electrons and is in need of three. So, in a pair, each nitrogen atom puts in three but holds back two. So, 2 held back + 6 shared = 8 = filled orbital. Since each atom ponied up three electrons, you guessed it, this is known as a triple covalent bond.

As for why we can breathe O2 but not O3, it's because . . . hm, you look a little wan. Why not take a nap and we'll discuss it another day?

August 22, 2005:  It is another day, after all: We'd like to thank an astute reader, b00p, for noticing and pointing out a small but significant error in this posting. The original posting stated that carbon has twelve protons, and that adding a single proton would give it thirteen, which would thereby make it nitrogen. In actuality, carbon's atomic weight is twelve, which consists mostly of six protons and six neutrons (the actual process for determining atomic weight is a LOT more complicated). Adding a proton would change the total number to seven, which would transform the carbon to nitrogen. The actual element with twelve protons is magnesium, and adding a proton would then make it aluminum, which has thirteen protons. To our casual readers, fret not; "adding a proton" is no simple matter. Alchemists were never able to turn lead into gold, and your chair is in no danger of turning into an aardvark.

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