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
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.
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
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
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.
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
Click here to return to the Chemistry Archives
Update: June 2012