MadSci Network: Chemistry
Query:

Re: If the fourth shell of an atom has 4 electrons

Date: Tue Jan 16 09:23:22 2001
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Chemistry
ID: 979512846.Ch
Message:

If the fourth outer shell of an atom has 4 electrons, is it more beneficial to lose 4 elctrons or gain 4?
The answer is NEITHER.

Every time you remove an electron from an atom, it takes more energy to remove the next: the ionization energy goes up with positive charge. And every time you add an electron to an atom, adding the next electron becomes less favorable: the electron affinity goes down with negative charge. So if an atom has to lose or gain more than one or two electrons to form an ionic compound, it won't normally do it. And in Groups IIIA, IVA and VA (or 13, 14 and 15 in the IUPAC system) almost all compounds are at least partly covalent.

Your question, of course, concerns elements of Group IVA.

While a number of compounds such as silicon nitride and tin(IV) oxide are named as if they are composed of ions (Si4+, N3- and Sn4+, O2-) they are actually covalent network compounds, in which electrons are shared pairwise between atoms. Other "ionic compounds" are actually molecular covalent compounds, like the Group IVA chlorides (CCl4, SiCl4, GeCl4, and SnCl4--PbCl4 exists but decomposes to PbCl2 and Cl2 at 50° C).

Truly ionic carbides and silicides do exist, but they do not contain quadruply-charged atoms. Instead, they contain molecular ions. For example, calcium carbide does not contain isolated C4- anions, but rather C22-.

Ionic compounds do exist in which charges of more than two units are confined to a single atom, but the only such compounds which are common are nitrides such as lithium nitride, Li3N.
Formally, of course, you can have either oxidation (E4+, such as SiO2) or reduction (E4-, such as CH4) but such compounds are, as I said, typically covalent rather than ionic. Whether such compounds are considered oxidized or reduced depends on the electronegativities involved.

Group IVA elements appear to have a preference for oxidation in that their hydrides--formally compounds of E4---are thermodynamically (and often kinetically) unstable in favor of the oxides. But this is simply a preference for the strong bond to oxygen; Group IVA chlorides are also unstable in favor of the oxides.

For more information, consult Greenwood and Earnshaw's Chemistry of the Elements or a text on main-group chemistry.

Dan Berger
Bluffton College
http://cs.bluffton.edu/~berger



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