Oxidation state in compounds

Finding the metal's oxidation state from a formula

1. Properties of iron

2. No charge written

Fe³⁺ Fe X^{3 +}

shows its charge. In

Fe2O3 Fe X_{2} O X_{3}

no charge is written, yet the book calls it iron (III) oxide.

Pick the right answer

Predict: in

FeO FeO

, oxygen counts as −2. What does iron count as?

A. +2
B. −2
C. +1
D. 0

Explanation

A neutral compound has a total charge of zero. With oxygen at −2, iron in

FeO FeO

counts as +2, because +2 + (−2) = 0.

3. Oxidation state

Oxidation state

The charge an atom has, or seems to have, in a compound or ion. It is written with a sign, like +2 or −1.

Particle	Oxidation state
An element on its own (Fe, $H2 H X_{2}$ )	0
A simple ion ( $Fe³⁺ Fe X^{3 +}$ , $Cl- Cl X^{-}$ )	its charge (+3, −1)

The two easy cases

Simple ions are easy: read the charge. In a compound, you calculate it.

4. Worked: iron in Fe₂O₃

In a compound	Value
O	usually −2
Na, K	+1
Cl in chlorides	−1
$SO4 SO X_{4}$ group (as a whole)	−2
Sum of all oxidation states	0

Rules you need in Chapter 1

Step 1: Call iron x

Two Fe atoms give 2x. Three O atoms give 3 × (−2).

Step 2: Make the sum zero

2x + 3(-2) = 0 \;\; 2x = +6 \;\; x = +3

Step 3: Name it

Iron is +3, so

Fe2O3 Fe X_{2} O X_{3}

is iron (III) oxide. The Roman numeral is the metal's oxidation state.

5. Balance chromium in CrCl₃

Cl in a chloride counts as −1. Move the slider until the sum for

CrCl3 CrCl X_{3}

is zero.

oxidation state of Cr 0

3 Cl atoms -3

sum: x + 3(−1) -3

6. Your turn

Fill in the blanks

KMnO4 KMnO X_{4}

, the oxidation state of Mn is _____.

Explanation

+1 + x + 4(−2) = 0, so x = +7.

Need a hint?

K counts +1 and each O counts −2.
Four O atoms give −8.
+1 + x − 8 must equal 0.

Pick the right answer

What is the name of

Fe2(SO4)3 Fe X_{2} (SO X_{4}) X_{3}

A. iron (III) sulphate
B. iron (II) sulphate
C. iron (VI) sulphate

Explanation

Three

SO4 SO X_{4}

groups give −6; two Fe atoms share +6, so each is +3.

7. Summary

Element alone = 0; simple ion = its charge.
In a compound the oxidation states add up to 0: O = −2, Na and K = +1, Cl in chlorides = −1, $SO4 SO X_{4}$ = −2.
The Roman numeral in a name is the metal's oxidation state: iron (II) = +2, iron (III) = +3.

Next in Chapter 1: the book's table lists each metal's oxidation states with its compounds.

Can you explain why iron is +3 in

FeCl3 FeCl X_{3}

but +2 in

FeCl2 FeCl X_{2}

Reveal

Cl in chlorides counts as −1. In

FeCl3 FeCl X_{3}

three Cl give −3, so iron is +3; in

FeCl2 FeCl X_{2}

two Cl give −2, so iron is +2. The sum is 0 each time.

Concepts in this lesson