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Name: Bruce E.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: 9/19/2004

I know that the electron configuration of chromium is [Ar]4s15d5. Why are the preferred ions of chromium Cr+2 and Cr+3 as opposed to Cr+1?

Many elements, such as chromium has electronic states that are not at a much higher state than the ground state. In the presence of other atoms electrons are "promoted" to these low energy states favorably because the energy gained in forming bonds exceeds the energy "cost" of "promoting" one or more electrons. Carbon is the classic example: Its ground state electronic configuration with its 6 electrons is: 1S2, 2S2, 2P. From this one would "expect" carbon to have an oxidation state of 2, which of course in seldom if ever does.

In the case of Cr the energy levels of its outer shell are: 3D5, 4S1 ( 0 cm^-1); 3d5, 4S1 (7593 cm^-1). This is due to the more stable anti parallel and less stable parallel spin orientation of the S and D electrons. The next energy level has a configuration 3D4, 4S2 which involves 4 sub energy levels centered at about (8000 cm^-1). This results in a multitude of possible bonding states accessible to Cr. You can find the energy levels of a lot of atoms and ions at the NIST web site: Look for energy levels of atoms on that home page.

Vince Calder

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