06.Periodicity of Behavior;
       Sodium Through Argon
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       The Acidic Nonmetals: P, S, and Cl

The larger silicon atom can accommodate four oxygens in silicic acid, H4SiO4, and the silicates. In aqueous solution, silicic acid is extremely weak and hardly ionized at all.

This is because silicon is less electronegative than carbon, and pulls the four negative charges in the silicate ion only weakly toward itself.

The negative charges remain mainly On the oxygens of the SiO44- ion, where they are able to attract protons easily and form undissociated sificic acid molecules.

Also, SiO44- is not stabilized by delocalization of electrons. Each electron pair in the ion is pinned down either in an oxygen lone pair or a Si-O single bond.

Aside from illustrating the beginning of a trend, silicic acid is of minor importance. The elements P, S, and Cl do form important oxyacids, each with four oxygens around the central atom.

These acids are phosphoric acid, H3PO4, sulfuric acid, H2SO4, and perchloric acid, HClO4.


Each of these is capable of losing all of its protons to form, respectively, the ions shown above: phosphate, PO43-; sulfate, SO42-, and perchlorate, ClO4-. As with carbonate and nitrate ions, no simple single-and-double bond model is entirely satisfactory for these ions.

Models can be drawn with single and double bonds to O, because it is possible with these larger atoms to put more than four electron pairs around the central atom.

However, such models suggest that not all the bonds to oxygen are the same length, which is in disagreement with experimental results.

If this difficulty is avoided by making a model with one single bond to each O, then the central ions wind up with positive charges, an undesirable state of affairs.

The best description of reality is to say that negative charge is delocalized over the entire ions, with the X-O bonds being approximately 11/4 bonds in phosphate, 11/2 bonds in sulfate, and 13/4, bonds in perchlorate.

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