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Name: Jasmine
Status: student
Grade: 9-12
Location: WI
Country: USA
Date: October 2008

Why are gases diatomic? Specifically, why is oxygen always O2?

Not all gases are diatomic, although some of the more common ones like N2, O2, and the halogens X2 (where X=F,Cl,Br,I). In contrast, the noble gases are single atoms, and N2O, NO2, O3, and H2O are triatomic. Volatile organic compounds may contain many atoms in the gaseous state. An example is hexane C6H14. There are thousands more examples.

To carry the explanation further, the structure of molecules in the gas state (or vapor) depends upon the temperature. A molecule that is polyatomic at low temperature may form species containing fewer atoms at sufficiently high temperature, and some substances that are nonvolatile at lower temperature, e.g. CaCl2 (solid crystal), becomes gaseous CaCl2 at sufficiently high temperature -- again, there are many other examples.

The composition of the gas phase of a substance can get quite complicated at high temperature. And at sufficiently high temperature ALL GASES ARE MONATOMIC.

Vince Calder

I am afraid to say that your question contains at least two errors - Firstly gasses are not always diatomic, and oxygen is not always 02.

Many gasses are diatomic because it is a convenient form which allows the atoms involved to share electrons to achieve filled outer shells - a 'preferred' state.

(I put 'preferred' in quotation marks because atoms have no will and cannot choose or have preferences - however the atom does achieve a state of lower energy if the outer electron shell is filled, so 'preferred' is an acceptable way of saying it is the state toward which the laws of physics will lead. )

In Chlorine and fluorine there are seven electrons in the outer shell, and each wants to have eight. By 'sharing' an electron from each atom, then both atoms can have six electrons exclusively and two shared ones - a total of eight. Sharing electrons is the basis of covalent bonding. Oxygen has six electrons in the outer shell, when it wants 8. If two oxygen atoms share two electrons in the bond, then each can have eight. We have a double bond. Nitrogen does the same by sharing three electrons each - we say there is a triple bond, and that is one of the reasons why nitrogen is so much less reactive than oxygen.

There are many gasses which are not diatomic though - Water vapour is H2O, Ammonia is NH3. Carbon Dioxide is CO2 and Sulphur Dioxide is SO2. The noble gasses - Neon, Argon, etc are all monatomic.

and last but not least Ozone is a triatomic form of Oxygen - O3

So the answer I have to give to your question is - They are not, and it is not!

Nigel Skelton
Tennant Creek High School

Actually, as it turns out, not all gases are diatomic. Oxygen is a good example in itself, because it can also exist as ozone, which has the formula O3, which is triatomic.

The noble gases Helium, Neon, Argon, Krypton, and Radon are all monatomic.

Familiar gases that are triatomic include carbon dioxide, hydrogen sulfide, ozone, and sulfur dioxide.

Familiar gases containing more than three atoms include methane, ethane, propane, butane, ammonia, sulfur hexafluoride, and freons.

Richard Barrans, Ph.D., M.Ed.

Hello Jasmine,

Gases are diatomic when it is energetically favorable for individual atoms of the gas to chemically bind to other atoms. While most of the gas we normally encounter is diatomic (i.e. the N2 and O2 in our atmosphere), many gases are not diatomic. The reason you often find diatomic oxygen is that it is energetically favorable for some of the outer electrons to be shared between two oxygen atoms than it is for those two atoms to be separate. However, this bond and is often better satisfied by the oxygen atoms bonding to something else. This is the cause of oxygen's reactivity while something like N2 is more stable. Noble gases such as Helium, Argon, and Neon are monatomic.

However, you can find both monatomic Oxygen and oxygen molecules with 3 (or more) atoms of oxygen. In a sufficient vacuum, if you create liberate an oxygen atom from a molecule (the simplest idea being the ionization of diatomic oxygen!) it can persist as monatomic oxygen for a time until it meets another oxygen (or other atom/molecule capable of binding with oxygen). There are also oxygen molecules that contain more than 2 atoms of oxygen. O3, known commonly as Ozone, contains 3 oxygen atoms. Oxygen molecules with higher numbers have been made in labs, but not seen in nature (to my knowledge).

While O2 is quite reactive, the other varieties (allotropes) are even more reactive. O3 tends not to exist very long in many conditions. O4 or O takes even more effort to keep from reacting!


Michael S. Pierce

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