Valency and Oxidation number as function of electrons distribution
Using electron formulas, possible values of valency and oxidation numbers for chemical elements may be predicted.
In general case valency is equal to the quantity of unpaired electrons on the last (for s- and p-elements) and next to last (for d-elements) sub-levels.
According to this idea valency of Chlorine may be equal to I (normal state), III (the first excited state), V (the second excited state) and VII (the last, third excited state).
For estimation of oxidation number the theory of “octet configuration” must be used. It says that all atoms pressing towards to obtain the same configuration of the last electron shell are similar to noble (indifferent) gases. Why? Because the noble gases are the most stable chemical elements and they have 8 (excluding He) electrons on the last electron shell (ns2np6). Chemical elements may form configuration like noble gases giving up unpaired electrons to other atoms or joining them from other atoms. If after such operation atom has structure of last or next to last electron shell absolutely like noble gas it will be stable. If its configuration is not “ideal”, we may predict that such one is not stable and of course compounds included these atoms.
According to this theory, we may predict that in the first (normal) state atom of Chlorine may have three figures of oxidation number: -1 (when it joined one unpaired electron from any other atom); 0 (without interchange of electrons with other atoms) and +1 (when it lost one electron). Only the first figure of oxidation number will respond to an octet configuration of the last electron shell (3s23p6) and Cl-1 is a stable oxidation number for this chemical elements. Truly in nature this element exists only in the form of Chlorides where Chlorine has oxidation number –1.
Let’s consider the first excited state of Chlorine atom. We see, that there are 3 unpaired electrons on the last electron shell. What can be said about possible degree of oxidation? The first possibility – to join 3 electrons from other atom and to have oxidation number –3. This state in not stable, because atom has not octet configuration. If it lost 3 elections, its configuration is more stable, because atom has two pairs of electrons.
Analogously it may be shown that oxidation numbers of Chlorine in the second and third excited states will be equal to +5 and +7. Oxidation number +5 is more stable that +1 and +3, but the most stable compounds respond to oxidation number +7.
Note, that valency and oxidation number are the different notions. Oxidation is the virtual figure, mathematical value. For example, in molecules H2 and O2 oxidation numbers are equal to 0, but valency is equal to 1 and 2 correspondingly.
Date: 2015-01-12; view: 208