How to determine the oxidation state of elements in a compound

The fas­ci­nat­ing sci­ence of chem­istry has the con­cept of ox­i­da­tion state, which is a num­ber for for­mu­lat­ing ox­i­da­tion-re­duc­tion re­ac­tions.

To be con­cise, in this sci­ence the ox­i­da­tion state means the con­di­tion­al charge in the atom which los­es or gains elec­trons, and this fig­ure is a method of cal­cu­lat­ing the trans­fer of elec­trons. This num­ber is giv­en to one atom or group of atoms, and char­ac­ter­izes the num­ber of re­dis­tribut­ed elec­trons, and shows the prin­ci­ple of trans­fer of elec­trons as a re­sult of a cer­tain chem­i­cal re­ac­tion.

De­ter­min­ing the ox­i­da­tion state is easy and dif­fi­cult at the same time – it de­pends on the atoms and the mol­e­cules that con­sti­tute them. It of­ten hap­pens that the atoms of some chem­i­cal el­e­ments can have an ab­so­lute­ly dif­fer­ent ox­i­da­tion state.

In or­der to sim­pli­fy the process of de­ter­min­ing the ox­i­da­tion state, spe­cial sim­ple rules are used, and any­one who knows the ba­sics of chem­istry and math­e­mat­ics will be able to use them to find a cer­tain ox­i­da­tion state with­out dif­fi­cul­ty. We should al­ways re­mem­ber that of­ten the ox­i­da­tion state and the va­len­cy of the el­e­ment are equal to each oth­er.

This sci­en­tif­ic top­ic is wide­ly stud­ied at school, so in or­der to un­der­stand how to find an ox­i­da­tion state, we pro­pose that you read this ar­ti­cle.

The first stage: we de­ter­mine whether the chem­i­cal sub­stance is el­e­men­tary.

The ox­i­da­tion state of atoms which do not in­ter­act with oth­er atoms in any way as a re­sult of chem­i­cal pro­cess­es is zero.

This prin­ci­ple is ap­plied to a group of sub­stances which was formed from in­di­vid­u­al free atoms. This rule is also ap­plied for chem­i­cal el­e­ments which con­sist of di­atom­ic or poly­atom­ic mol­e­cules of just one el­e­ment.

For ex­am­ple, iron Fe and cal­ci­um Ca have an ox­i­da­tion state of zero, be­cause they con­sist of one el­e­ment that is not chem­i­cal­ly bond­ed with oth­ers, and so do poly­atom­ic mol­e­cules with the same type of atom, for ex­am­ple for ozone O₃ the ox­i­da­tion state will also be 0.

Cal­cu­lat­ing the ox­i­da­tion state in ion­ic com­pounds

The ox­i­da­tion state is iden­ti­cal to the charge on the atoms or group of atoms. This prin­ci­ple is ap­pli­ca­ble both for free ions and for those which are part of the struc­ture of chem­i­cal com­pounds.

For ex­am­ple, the ox­i­da­tion state of the chlo­rine ion is -1, and if we ex­am­ine chlo­rine in a chem­i­cal com­pound, for ex­am­ple in hy­drochlo­ric acid HCl, the ox­i­da­tion state of this el­e­ment will also be -1. As a hy­dro­gen ion has an ox­i­da­tion state of +1, the charge of the chlo­rine ion is -1, which means that its ox­i­da­tion state is -1.

Met­al ions can have many ox­i­da­tion states

We will ex­am­ine this us­ing the ex­am­ple of iron (Fe), be­cause its ion can have a charge of +2 and +3. The charge of ions of metal­lic el­e­ments can be de­ter­mined by the charge of oth­er ions in the chem­i­cal com­pound, and in writ­ing for­mu­las this charge is in­di­cat­ed by Ro­man nu­mer­als, for ex­am­ple iron (II) has an ox­i­da­tion state of +2. Here you’ll find stun­ning safe ex­per­i­ments with iron.

How do we find the ox­i­da­tion state in a com­pound?

As we have al­ready es­tab­lished, the com­pound should be neu­tral. We will ex­am­ine Al­Cl₃.

As we said above, the charge of the ions in chlo­rine is -1, and in this com­pound there are three chlo­rine atoms. Ac­cord­ing­ly, to com­pen­sate the mi­nus­es, the charge of alu­minum must be +3.

How do we de­ter­mine the ox­i­da­tion state of O₂?

When oxy­gen is in a free state (with­out en­ter­ing into a re­ac­tion with any el­e­ments), the ox­i­da­tion state is zero (like oth­er el­e­men­tary el­e­ments, in fact).

If oxy­gen is a com­po­nent of any hy­drox­ide, for ex­am­ple hy­dro­gen hy­drox­ide H2O2, it will have an ox­i­da­tion state of -1.

If oxy­gen in­ter­acts with flu­o­rine (F), it will have an ox­i­da­tion state of +2.

Let us ex­am­ine the ox­i­da­tion state of hy­dro­gen H

This chem­i­cal el­e­ment has an ox­i­da­tion state of +1 (ex­cept the molec­u­lar state of hy­dro­gen), but there are ex­cep­tion­al cas­es.

For ex­am­ple, in wa­ter H₂O the ox­i­da­tion state of hy­dro­gen will be +1, be­cause the ox­i­da­tion state of oxy­gen is -2, and so the whole com­pound, ac­cord­ing to the rules, has a neu­tral charge.

But if we take NaH, the ox­i­da­tion state of H will be -1, as sodi­um has a charge of +1.

How to de­ter­mine the ox­i­da­tion state of flu­o­rine (F)

Al­though the ox­i­da­tion state of chem­i­cal el­e­ments de­pends in most cas­es on nu­mer­ous fac­tors, flu­o­rine will al­ways have an ox­i­da­tion state of -1. This is be­cause flu­o­rine has a low elec­tri­cal neg­a­tiv­i­ty, i.e. F atoms un­will­ing­ly break free from their own elec­trons, but in­ten­sive­ly at­tract elec­trons of oth­er el­e­ments.

Rule: the sum of ox­i­da­tions states is equal to the charge of the chem­i­cal el­e­ment.

The sum of ox­i­da­tion states of all atoms of the com­pound should be neu­tral. With this rule, we can test whether or not we have made a mis­take in solv­ing a chem­istry prob­lem.

How do we de­ter­mine ox­i­da­tion states? Here’s some use­ful ad­vice which will help in solv­ing prob­lems:

The pe­ri­od­ic ta­ble will come in handy to make the cal­cu­la­tion pre­cise. You must learn to use it prop­er­ly and dis­tin­guish where met­als and non-met­als are lo­cat­ed.

To find the ox­i­da­tion state of met­als, which of­ten have sev­er­al cor­re­spond­ing val­ues, you must de­ter­mine them by the ox­i­da­tion states of oth­er atoms in the com­pound.

If you add up all the val­ues of ox­i­da­tion of atoms in a chem­i­cal bond, you will al­ways get a zero ox­i­da­tion state.

The high­est ox­i­da­tion state of an el­e­ment is de­ter­mined us­ing the pe­ri­od­ic ta­ble by the group in which it is lo­cat­ed.

Met­als in all com­pounds have a pos­i­tive ox­i­da­tion state.

In com­pounds with non-met­als, hy­dro­gen has an ox­i­da­tion state of +1, and an ox­i­da­tion state of -1 with met­als.

In com­pounds, oxy­gen has an ox­i­da­tion state of -2, apart from in H₂O₂, OF₂, K₂O₂.

The ox­i­da­tion states of non-met­als when joined to met­al atoms will al­ways be neg­a­tive, but on in­ter­ac­tions with atoms of non-met­als they may have a pos­i­tive or neg­a­tive ox­i­da­tion state.

To find the high­est ox­i­da­tion state in non-met­als, from the num­ber 8 sub­tract the num­ber of the group in which the el­e­ment is lo­cat­ed, and the high­est ox­i­da­tion state with a plus sign will be equal to the num­ber of elec­trons on the out­er lay­er. To find the num­ber of elec­trons in the out­er lay­er, look at the num­ber of the group in the pe­ri­od­ic ta­ble.