Acid hard

a hard acid is a Acide of Lewis whose center acceptor of electron is not very polarizable. A hard acid is thus generally not very bulky and very charged , formed of very electronegative and not very polarizable atoms. The opposite of a hard acid is a acid slackness whose center acceptor is on the contrary very polarizable. The criterion of hardness of an acid is used in the principle Hard and Soft Acid and Base . The criterion of hardness is also applicable for the bases. This principle makes it possible to determine affinity between acids and bases in reactions according to their hardness. Indeed a hard acid tends more to react with a hard base than with a soft base.

Examples of hard acids

The hard acids are generally cations. The loss of electron in a cation decreases the size of the cloud of the electrons and thus the cations are species not very bulky not very polarizable what corresponds to the definition of a hard acid. Cependement the majority of the cations having a raised atomic number are not hard acids. Their clouds of electrons are of too big size, they are species bulky and polarizable in spite of their loss of electrons. The hard acids having a raised atomic number are cations having a very high load, i.e. having lost a big number of electrons: Ti4+ titanium, Zr4+ Zirconium, Th4+ Thorium

See too

http://www.meta-synthesis.com/webbook/43_hsab/HSAB.html

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