Elements with a variable oxidation state

The top­ic of the ox­i­da­tion state of el­e­ments is con­sid­ered to be of the most im­por­tant in the study of chem­istry. For some el­e­ments, this fig­ure is con­stant, while for oth­ers it is vari­able. The ma­jor­i­ty of el­e­ments have a vari­able ox­i­da­tion state, in fact. This means that in dif­fer­ent com­pounds it will dif­fer. It’s im­por­tant to take this into ac­count in mak­ing chem­i­cal equa­tions. In or­der to de­ter­mine the ox­i­da­tion state of an el­e­ment cor­rect­ly, you should be guid­ed by sev­er­al main rules.

Main con­cepts

The ox­i­da­tion state is a for­mal val­ue. It as­sumes that all the bonds are ion­ic in the mol­e­cule, and helps to cal­cu­late the for­mal charge of the atom.

A vari­able ox­i­da­tion state is a val­ue that de­ter­mines the charge of the atom de­pend­ing on cer­tain con­di­tions. Un­der the im­pact of many fac­tors, the fig­ure may dif­fer for one and the same el­e­ment.

This is above all im­por­tant in prac­tice so that we can cor­rect­ly draw up the elec­tron bal­ance. This may of­ten be re­quired to bal­ance the co­ef­fi­cients in an equa­tion. Usu­al­ly, the ox­i­da­tion state in el­e­ments changes dur­ing the ox­i­diz­ing-re­duc­ing re­ac­tions. If one el­e­ment in the re­ac­tion has ox­i­dized, the oth­er must be re­duced. To put it more sim­ply, if one el­e­ment has giv­en up elec­trons, then an­oth­er el­e­ment must ac­cept them. Only el­e­ments in which a vari­able ox­i­da­tion state is pos­si­ble can take part in such trans­for­ma­tions. If an el­e­ment has the high­est ox­i­da­tion state, then it can only take elec­trons, while if it has the low­est, it can only give them up.

An ox­i­da­tion state can be pos­i­tive or neg­a­tive. It is in­di­cat­ed by Ara­bic nu­mer­als, with a plus or mi­nus sign ac­cord­ing­ly.

It is im­por­tant to take into ac­count the fact that the zero ox­i­da­tion state is not usu­al­ly in­clud­ed among the per­mis­si­ble states. The fact is that this val­ue will be pos­si­ble for ab­so­lute­ly any [el­e­ment](https://melscience.com/en/ar­ti­cles/10-cu­ri­ous-ques­tions-about-chem­i­cal-el­e­ments/. For ex­am­ple, if oxy­gen dis­plays an ox­i­da­tion state of -2 (in the vast ma­jor­i­ty of cas­es) or 0, we still say that it has a sin­gle ox­i­da­tion state.

Fre­quent­ly, the ox­i­da­tion state and va­lence in com­pounds will be the same num­ber. The dif­fer­ence is only in the sign (va­lence can­not be neg­a­tive). So the task may be sim­pli­fied some­what.

Rules for de­ter­min­ing ox­i­da­tion state

It is of­ten quite dif­fi­cult to de­ter­mine the ox­i­da­tion state in an el­e­ment when it is vari­able. Be­sides many rules, there is the sim­plest method of de­ter­min­ing this fig­ure in cer­tain com­pounds. The ox­i­da­tion states of el­e­ments in a com­pound must be equal to zero when added to­geth­er. So if it is +6 for one el­e­ment, for an­oth­er it must be -6. Thus it is eas­i­er to se­lect the el­e­ment with a con­stant ox­i­da­tion state in a com­pound, then mul­ti­ply this num­ber by the num­ber of el­e­ments in the el­e­ment. Then we di­vide this fig­ure by the num­ber of atoms of the oth­er el­e­ment, and write down this num­ber with the op­po­site plus or mi­nus sign.

It is ab­so­lute­ly unim­por­tant how many el­e­ments there are in the com­pound. Even if there are sev­er­al of them, the sum of their ox­i­da­tion states must still equal zero. For cer­tain groups (for ex­am­ple acid residue), this fig­ure is cal­cu­lat­ed gen­er­al­ly – in a com­pound the group is per­ceived as one el­e­ment, with a cer­tain ox­i­da­tion state.

We may also state sev­er­al main rules which al­low us to de­ter­mine the ox­i­da­tion state of an el­e­ment:

• sim­ple sub­stances it is al­ways equal to zero;
• the ox­i­da­tion state in met­als is pos­i­tive;
• the high­est ox­i­da­tion state for any el­e­ment is the pos­i­tive val­ue of its group num­ber;
• the low­est state for met­als is al­ways 0, and for oth­er el­e­ments it is equal to 8 mi­nus the num­ber of the el­e­ment group);
• for a sin­gle ion the ox­i­da­tion state is equal to its charge;
• the ox­i­da­tion state of hy­dro­gen is +1 (with the ex­cep­tion of hy­drides), +3 for alu­minum, -1 for flu­o­rine, and -2 for oxy­gen;
• for al­ka­line earth met­als the ox­i­da­tion state is +2;
• for al­ka­line met­als the ox­i­da­tion state is al­ways +1.

Al­though the rules are sim­ple, a num­ber of dif­fi­cul­ties may arise. This is main­ly be­cause some el­e­ments dis­play a dif­fer­ent ox­i­da­tion state de­pend­ing on the el­e­ments that they form com­pounds with:

• in com­pounds of non-met­als with non-met­als, the neg­a­tive ox­i­da­tion state will be found in the el­e­ment which has greater elec­troneg­a­tiv­i­ty;
• in com­pounds with met­als, the non-met­al of­ten has a neg­a­tive ox­i­da­tion state.

An ad­di­tion­al dif­fi­cul­ty is that there are ex­cep­tions to many rules. For ex­am­ple, even hy­dro­gen can have an ox­i­da­tion state of -1 in some cas­es. An el­e­ment will have a neg­a­tive ox­i­da­tion state if it forms a com­pound with met­als.

Click here to pass in­ter­est­ing quizzes about chem­i­cal el­e­ments.

In de­ter­min­ing the ox­i­da­tion state of el­e­ments in com­pounds where sev­er­al el­e­ments with vari­able ox­i­da­tions states are present, it is im­por­tant to try to sim­pli­fy the task. For ex­am­ple, you can try to di­vide the mol­e­cule into sim­pler com­po­nents. In a mol­e­cule, we may of­ten find acid residue, for ex­am­ple, the ox­i­da­tion state of which can eas­i­ly be found in ta­bles, or cal­cu­lat­ed from the for­mu­la of acid. Al­though in some cas­es acid residue may also have a dif­fer­ent ox­i­da­tion state de­pend­ing on the type of acid, but this is en­coun­tered ex­treme­ly rarely.

These changes are pri­mar­i­ly ex­plained by the fact that in a com­pound with dif­fer­ent el­e­ments, dif­fer­ent chem­i­cal prop­er­ties may be dis­played. We should take this into ac­count. For this rea­son, it is some­times dif­fi­cult to pre­dict pre­cise­ly the spe­cif­ic ox­i­da­tion state of an el­e­ment in all cas­es with­out cal­cu­la­tions.