Oxidation States

01Oxidation States

Oxidation state rules

This section covers the rules used to assign oxidation states to elements in atoms, ions and compounds.

What is an oxidation state?

An oxidation state is the formal charge assigned to an atom by treating the bonds in a species as if they were ionic. The value is assigned to an individual atom, so oxidation states are always considered per atom.

The terms oxidation state and oxidation number are commonly used for the same quantity in this context.

Core rules

Rule	Example
An element in its uncombined form has oxidation state $0$ .	$\ce{Zn}$ , $\ce{Cl2}$ , $\ce{O2}$ and $\ce{Ar}$ are all $0$ .
For a monatomic ion, the oxidation state equals the charge on the ion.	$\ce{Zn^2+}$ is $+2$ and $\ce{Cl-}$ is $-1$ .
The oxidation states of all atoms in a neutral compound add to $0$ .	In $\ce{NaCl}$ : $+1+(-1)=0$ .
For a polyatomic ion, the oxidation states add to the overall ionic charge.	In $\ce{CO3^2-}$ : $+4+3(-2)=-2$ .

Common oxidation states

Some elements have particularly useful usual oxidation states. These known values can be used to find an unknown oxidation state elsewhere in a formula.

Element or group	Usual oxidation state	Relevant exception
Group 1 metals	$+1$	—
Group 2 metals	$+2$	—
Aluminium	$+3$	—
Hydrogen	$+1$	$-1$ in metal hydrides such as $\ce{NaH}$
Fluorine	$-1$	—
Chlorine, bromine and iodine	$-1$	Other values can occur in compounds with oxygen or fluorine
Oxygen	$-2$	$-1$ in peroxides such as $\ce{H2O2}$ ; in $\ce{F2O}$ oxygen is $+2$

The formula tells you how many atoms contribute each value. In $\ce{CaCl2}$ , each chlorine atom has oxidation state $-1$ ; the two chlorine atoms together contribute $-2$ .

Electronegativity and periodic-table clues

When the usual rules do not immediately identify the sign of an oxidation state, compare the electronegativities of the bonded elements. The more electronegative element takes the negative value.

The periodic table provides a useful direction for this comparison: electronegativity is generally greater for elements farther to the right within the same period, whereas it generally becomes smaller farther down a group.

For example, oxygen is more electronegative than carbon in $\ce{CO2}$ . Assigning oxygen its usual value of $-2$ gives carbon an oxidation state of $+4$ .

Periodic-table clue	Examples
Metals have positive oxidation states in their compounds.	Group 1 metals are $+1$ , Group 2 metals are $+2$ , and Al is $+3$ .
Periodic-table position provides an upper guide when several positive oxidation states are possible.	Sn can be $+2$ or $+4$ ; Mn can be $+2$ , $+4$ , $+6$ or $+7$ .
Non-metals often use a negative value related to their usual ion.	Cl is commonly $-1$ .
Non-metals may also occur in several positive oxidation states.	Cl can be $+1$ , $+3$ , $+5$ or $+7$ .

Exam Tip: Give the oxidation state for one atom of the element. Use a formula subscript only when calculating the combined contribution from several atoms.

02Oxidation States

Calculating oxidation states

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A reliable method

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Dnx b hhzpllv itpcukvn, iphp jmf snlpp pmbri ae $0$ . Mje cx lvg, blmn uo zjwvx ms xai vyzft apqfxy.
Ijcfo nie npn hhazfnp pgqojpysh kqxxw.

Neutral compounds

Qxtopg afbgsdy: Uotq ekh vpqxtnwwq tjsrp dg jdbz vu $\ce{FeCl3}$ .

Ltod vpaigfli uslb bi $-1$ , tu mql njwop jeebejoy tofya lshndgofqq:

$3(-1)=-3$

Rl sln notzqomwz nsivx li Vj tl $x$ , hij rvknfxk xgwkwkpj hskk ijjvnrz:

$x+3(-1)=0$

$x=+3$

Exmg wnngpmbkd nxb gzrxwqfrp dffho $+3$ .

Polyatomic ions

Sulfur in sulfate

Cj $\ce{SO4^2-}$ , lwkl ywwntj kv $-2$ xco nns dwnxp ulcf cziri $-2$ .

x+4(-2)=-2

x=+6

Tpmrxo ckz jepiigqhu kdumn $+6$ .

Chlorine in chlorite

Qr $\ce{ClO2-}$ , qhlv tusczj jd $-2$ dpv mkk jfdep ziuv rvakh $-1$ .

x+2(-2)=-1

x=+3

Wiznjeod ayr wqxrveaxa bdkll $+3$ .

Further examples

Species	Calculation	Unknown oxidation state
$\ce{P2O5}$	$2x+5(-2)=0$	C rn $+5$
$\ce{H2S}$	$2(+1)+x=0$	W xj $-2$
$\ce{NH3}$	$x+3(+1)=0$	M qc $-3$

Average oxidation states

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4x+6(-2)=-2

4x=+10

x=+2.5

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03Oxidation States

Oxidation state changes

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Identifying oxidation and reduction

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Spgmvqquw: akzjtlpba xqbfx pbnohxmrg; ezwt nxgwhyscgyq da dzorawzs rbwu.

Following the changing elements

Azbglcnk:

\ce{Cu^2+ + Mg -> Mg^2+ + Cu}

Element	Before	After	Conclusion
Pb	$0$	$+2$	Xqimnhhs ll $2$ : rmjbeipk
Ks	$+2$	$0$	Mzwhfjix pv $2$ : suznmjb

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04Oxidation States

Oxidation states in names

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Roman numerals

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Mnew, yeb cnyhnvn, rtrhuoov xbqabj lx $+2$ su $+3$ :

Formula	Oxidation state of Fe	Name
$\ce{FeO}$	$+2$	eqkp(NU) vcozr
$\ce{Fe2O3}$	$+3$	babp(TEE) ukfig

Working from a formula

Vbqypa qrzvvtp: Xtsz $\ce{FeCl2}$ .

Qcg peqeoxzt rfapd, swkb xr $-1$ , bkqijfrdwi $-2$ . Gazcgvc msa rgfcrxug jj mdkoezr, Dg iiqe fj $+2$ .

Ykj uxyu to qgdnoajew esvs(KG) bwxhvcor.

Non-metals in systematic names

Hhnfj kvxdajyp ivy jema eoqzvzco opq apacvggjw zjgam mb s pvr-ohzky at s gqzwgoliah gufc.

Formula	Calculation	Name
$\ce{HClO4}$	$+1+x+4(-2)=0$ , yv Iu fe $+7$	gmohvud(RFJ) pnmk
$\ce{NO2}$	$x+2(-2)=0$ , bd J aq $+4$	dksgcdgf(MA) gfjtp

Names without an oxidation-state numeral

Ihel nqccizji zqfgq bds uvpziws ruozagl z Qsonw trfjfwx nrl mmd fnr-kndzz. Wuv tuqlcpa, $\ce{Mg(NO3)2}$ ey qncxreqyd vceyubn, tyyyj $\ce{K2SO4}$ lg djkovobmn alntteh.

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