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The bonding in carbon dioxide (CO2): all atoms are surrounded by 8 electrons, according to the octet rule. CO2 is thus a stable molecule.

The octet rule is a simple chemical rule of thumb that states that atoms tend to combine in such a way that they each have eight electrons in their valence shells, giving them the same electronic configuration as a noble gas. The rule is applicable to the main-group elements, especially carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium or magnesium. In simple terms, molecules or ions tend to be most stable when the outermost electron shells of their constituent atoms contain eight electrons.

Contents

Explanation in quantum theory

The quantum theory is used to explain the valence electron energy shells. In short, an element's valence shell is full and most stable when it contains eight electrons, corresponding to an s2p6 electron configuration. This stability is the reason that the noble gases are so unreactive, for example neon with electron configuration 1s2 2s2 2p6. (Helium is an exception as explained below).

Note that a "full shell" means that there are the eight electrons in the valence shell when the next shell starts filling, even though higher subshells (d, f, etc.) have not been filled. There can be at most eight valence electrons in a ground-state atom because p subshells are always followed by the s subshell of the next shell. This means that once there are 8 valence electrons (when the p subshell is filled), the next additional electron goes into the next shell, which then becomes the valence shell.

A consequence of the octet rule is that atoms generally react by gaining, losing, or sharing electrons in order to achieve a complete octet of valence electrons. Reaction of atoms occurs primarily in two ways: ionically and covalently.

Some of the atoms for which the octet rule are most useful are:

Exceptions

History

In the late 19th century it was known that coordination compounds (formerly called “molecular compounds”) were formed by the combination of atoms or molecules in such a manner that the valencies of the atoms involved apparently became satisfied. In 1893, Alfred Werner showed that the number of atoms or groups associated with a central atom (the “coordination number”) is often 4 or 6; other coordination numbers up to a maximum of 8 were known, but less frequent. In 1904 Richard Abegg was one of the first to extend the concept of coordination number to a concept of valence in which he distinguished atoms as electron donors or acceptors, leading to positive and negative valence states which greatly resemble the modern concept of oxidation states. Abegg noted that the difference between the maximum positive and negative valences of an element under his model is frequently eight.[1] Gilbert N. Lewis referred to this insight as Abegg's rule and used it to help formulate his cubical atom model and the "rule of eight" which began to distinguish between valence and valence electrons.[2] In 1919 Irving Langmuir refined these concepts further and renamed them the "cubical octet atom" and "octet theory".[3] The "octet theory" evolved into what is now known as the "octet rule".

See also

References

  1. ^ Abegg, R. (1904). "Die Valenz und das periodische System. Versuch einer Theorie der Molekularverbindungen (The valency and the periodical system - Attempt on a theory of molecular compound)". Zeitschrift für anorganische Chemie 39 (1): 330–380. doi:10.1002/zaac.19040390125. http://dx.doi.org/10.1002/zaac.19040390125. Retrieved 2009-11-10.  
  2. ^ Lewis, Gilbert N. (1916-04-01). "THE ATOM AND THE MOLECULE.". Journal of the American Chemical Society 38 (4): 762–785. doi:10.1021/ja02261a002. http://dx.doi.org/10.1021/ja02261a002. Retrieved 2009-11-10.  
  3. ^ Langmuir, Irving (1919-06-01). "THE ARRANGEMENT OF ELECTRONS IN ATOMS AND MOLECULES.". Journal of the American Chemical Society 41 (6): 868–934. doi:10.1021/ja02227a002. http://dx.doi.org/10.1021/ja02227a002. Retrieved 2009-11-10.  
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Simple English

The Octet rule is a general rule of thumb that applies to most atoms. Basically, it states that every atom wants to have eight valence electrons in its outermost electron shell.

Background

According to the Bohr model, an atom consists of a nucleus of protons and neutrons orbited by a number of electrons. In an ordinary atom, the number of electrons equals the number of protons. If it has more or fewer electrons, it is an ion. In addition, the electrons orbit in electron shells. Each shell can only contain a certain number of electrons before new electrons must migrate to the next shell or "energy level." Starting with the third energy level, however, electrons sometimes move up a level even before the current one is full.

The rule

Every atom's tries to have 8 electrons in its outermost (or valence) shell. This may require them to give up, share, or take electrons. As stated above, the number of electrons normally equals the number of protons. This means that some atoms are "happier" than others. For example, neon, as with other noble gases naturally has 8 valence electrons. (10 total - 2 in the 1st energy level=8 in the 2nd) Not surprisingly, neon hardly ever reacts with anything. (Neon lights require high voltages to work.) By contrast, the atoms in the Alkaline metals column, such as sodium, have one valence electron, with a layer containing 8 electrons in the shell below. That means that if sodium only has to give up that one electron in order to complete the octet rule, and does so if given the chance. This is why sodium is so chemically reactive.


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