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Why do we not assign oxidation numbers to ionic compounds?

Answer 1

Because MOST of the time, the identity of the ionic compound is specified by the regular oxidation states expressed by each ion in the compound.

Alkali metal salts typically form a #I^+# ion; and alkaline earth salts typically form a #II^+# ion. Halogens, #"Group VII elements"#, typically form an #X^-# ion; and chalcogens, #"Group VI elements"#, typically form an #E^(2-)# ion.

Given these charges, an halide anion, will form a #MX# salt with the alkali metals, and an #M'X_2# salt with the alkaline earths. The salt must NECESSARILY be neutral. And likewise for the chalcogen salts, #M_2E#, and #M'E# etc.

Here, all we've done is charge in the proper balance.

On the other hand, there are some situations where the oxidation state of the metal is ambiguous, in that the metal can adopt 2 oxidation states. #"Ferric salts"# of #Fe^(3+)#, and #"ferrous salts"# of #Fe^(2+)# are good examples. In this scenario the oxidation states must simply be known. More complex anions, e.g. #"chlorates"#, #"perchlorates"#, are sometimes distinguished by the oxidation state of the halogen: #"chlorate "=Cl(+V); "perchlorate "=Cl(+VII)#. Depending on where your are A level or undergrad, you have to make the determination as to what you need to know.

Consequently, it ought to be relatively simple to ascertain the molecularity and composition of a particular ionic compound.

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Answer 2

Peyton Adams

We do not assign oxidation numbers to individual ions in ionic compounds because oxidation numbers are assigned to atoms in covalent compounds or molecules, where electrons are shared between atoms. In ionic compounds, ions are formed through the transfer of electrons from one atom to another, resulting in ions with a fixed charge. Therefore, oxidation numbers are not relevant for individual ions in ionic compounds.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.